When we ordered our new bike, I asked for eyelets on the rear triangle to be able to mount a rear rack. They did not get installed. We have an upcoming supported multi-day ride where we will want to carry a small amount of stuff (raingear, snacks, etc.) in a rear rack. Is there a good way to temporarily mount a rack to the rear of a CF frame without modifying or damaging the frame?

Have you thought about one of the seat post mounted racks? We've used one on occasions like this and they can hold more weight then you would think. We did a two day little tour using a trunk bag and our seat post rack and it worked great. I'm not remembering what seat post you are you using for your stoker, I was thinking it was a Body Float.

Seat post rack for me.

We used a Revelate Pika, which is a bikepacking saddle bag and it held a lot of stuff. Just make sure you have enough distance between the stoker seat and the top of the rear wheel.

hardware store

Seconded. Be sure to get the rubber coated metal ones, the nylon ones won't hold up to weight and bumps.

Any reason p-clips can't be used on CF stays? Agree that rubber-coated ones would be the way to go.

I don't recommend seatpost mounted racks (because they sway annoyingly) other than this one:
https://www.arkel-od.com/en/arkel-randonneur-rack.html

A few items to consider:
Seatpost collar mounting holes:
Axiom Trekk Seat Collar w/Rack Eyelets in Tree Fort Bikes Seatpost Collars

Quick-release skewer mounted rack (might not play nice with tandem spacing):
Axiom Streamliner Road DLX Rear Rack in Tree Fort Bikes Rear Mount Racks

Quick-release skewer mounted rack with lots of options:
Pioneer Rear 29'' Plus Mech. Disc. Hub-Mounted 5mm Quick Release with Clamps - Old Man Mountain specializes in Racks designed to work on all bikes.

I'd also consider frame bags and/or enormous seat bags. Ibera and Planet X have super cheap ones.
Front racks are good, too.

also less likely to slip - and if you can't find a size that fits exactly you can wrap the bike part w hockey, tape, electrical tape, or those rubber bits they sell with bike lights, or I guess a cut piece of tire tube

I've never read that anywhere

I agree. this one actually twisted my saddle side to side as I rode. so I built support bars



many years (& bikes) later I recently put it into service again. while it works great for me, it sits much higher than when you can mount a rack lower on the bike frame



a pro bike shop can certainly set you up so it doesn't look amateurish

my road bike rack is much lower (where it should be)



that bike's top mount



and here's the bottom. wing nuts aren't always my 1st choice. must have been what I had on hand. but if they loosen while riding they're easy to tighten :-)

Here is the rack we have used for 1000's of miles of supported tours with the slide on topeak bags. Plenty big enough to hold two jackets and snacks etc. Rack has never slipped or had any issues. Very light and easy to pack in travel cases. We Use it on Thomson master piece seat posts.
https://www.amazon.com/Topeak-Beam-C.../dp/B000ZKFXK2

expensive but I like how long it is. I had to build an extension rail for mine

Have you considered using a frame bag? On many tandem frames you can put a frame bag attached to the stoker's top tube, and captains' seat tube that is not in either riders way.

A moderate size frame bag, and a decent sized saddle bag would likely be more than adequate to carry the amount of stuff you're describing.

Easier, cheaper, and lighter than a rack and panniers, or trunk, and carries the weight lower than a rack and trunk combination.

Thanks for all the suggestions. Some I thought of already, others are new (to me) ideas.

you have another option for your Calfee Use a seat post clamp like this one Problem Solvers

Then you can use the machine screw that joins the seat stay and chain stay to mount a conventional rear rack. This is my solution on our Calfee adventure tandem frame

We have used this Topeak post-mounted pod with great success in carrying almost exactly what the OP asked about: rain gear, snacks, sunscreen and the occasional bottle of wine.
It does NOT sway too much with our cargo load.

We do have a clamp-on rack that clamps to the seatpost. I have not yet tried it but the seatpost on the tandem is CF. Hopefully this won't damage the seatpost. I also got some padded stainless steel cable clamps that I can try with a standard rack.

We've done it with a clamp on rack and panniers attached (easily more than 20 pounds total weight) without issue.

I'm sure you'll see warnings from CF manufacturers, and various posts on the internet strongly advising against doing so.

But if you think about it on a tandem, you're clamping the stoker's stem to the Captains' carbon fiber seat post.

The force a captains seat post gets from a stoker yanking on the stoker bars in an all out sprint has to be way more than from the rack in that picture.

Also, the obvious concern in clamping things onto carbon is a concern about crushing. (this is why you don't want to clamp aero bars on to most carbon fiber handlebars) However, a seat post is designed to take clamping forces along its entire length, given that it can be clamped at any point above the minimum insertion depending on the rider's height.

So don't clamp the rack on tighter than you would tighten the seat collar around the seat post and you should be fine.

The rack we have comes with various size rubber sleeves to get the fit to the seat post right, prevent scratching the post, and reduce the amount of clamping force necessary to hold the rack in place.

You could switch-out the seat post for this trip with an aluminum one if you were concerned about the clamping force. One thing I did with our seat-post mounted rack was cut a short piece of 18-23c inner tube which I slipped over the seat post which worked as a buffer for the clamp on our at the time brand new Thud buster seat-post. We didn't experience and sway or shifting of the rack for our little two-day tour.

With a CF frame it wouldn't be my thinking that the frame will need shimming, it would be my concern that the P-clip would not open wide enough, or have enough clearance to be properly secured by the mounting bolts. But IMO of all the methods presented, p-clips that will fit the tubing, or something very much like them is the best, maybe only practical way.

is CF frame tubing not round? regardless, I agree matching frame diameter to clamp diameter is a challenge. there are lots of clamps to choose from. also clamps have some degree of tolerance for differences. I think they all open up pretty well w/o breaking.

for this light mount on a thin rack, I used some weird perforated strapping I had on hand. I forget but I may have had to drill a new hole on one side so that after wrapping it, the holes aligned in such a way that there was enough tension to hold it tight. not handsome, I admit & certainly not suitable for a heavy load bearing rack mount. but as a light mount, kinda got lucky & actually holds up after several years of use w/o slippage

Are you 100% certain Calfee did not install the correct threaded rack mount w/bolts?

The first time we ordered with the rack mount option, I was certain it was not done as it appeared to be the same as on a previous frame. After some consternation and reassurance from Calfee, I then proceeded to unscrew the installed bolts only to find there was indeed the correct rack mount installed. Oops. My bad.

I didn't look carefully but I was thinking there would be one on each dropout plus a pair somewhere near where the seatstays meet the seat tube. I'll look again.

Use the seatpost clamp I suggested above and seat stay/chainstay threaded rack mount bolts and you are set for a conventional rear rack that does not affect your paint or other carbon in any way