how's this?

I made my own blinkie from an old reflector and Cateye seatpost light. Attached it to my rack with some silicon adhesive...great stuff! Its been on there for the past 2yrs now.

That's fantastic! I even have 90% of a roll of that metal tape left over from earthquake-strapping my water heater. AND, I already used that stuff to jimmy up little extendo-holes, reaching up from my rear-axle rack-mount screw-holes, so that when I use a seatpost rack with panniers, the short hook-straps can reach something and cinch tight. (does that make sense?)

This is the solution I will use, much easier than a little wooden block full of holes. And probably sturdier too.

Hey, for that first pic, it looks like the metal tape is pretty loose around the rack-rail there; is there something I'm not seeing that is making it grab tight and not wobble around or slide down?

PB blinkie 5 on the back of my delta mega rack. over a hundred night time miles in the last two weeks and it is still on and blinking.

roll the outermost, horizontal metal bar with your favorite material. duct tape, laminated paper, plastic sheets, vinyl, aluminum foil, leather, electrical tape, old tubes (from wheels), etc. Make the roll as thick as your seat post. now you can strap your existing lamp to the roll.

Ok. So what about his suggestion of putting the light on your helmet? If your helmet facilitates that, it's about as easy and unblockable (new word?) as you'll find.

That's not a bad idea, but for my rack, the "outermost, horizontal metal bar" has a vertical metal tab welded to it right in the middle (with two holes for mounting a rear light), and there's very little room for rolling on either side. For reference, my rack looks a lot like this one.

That's not a bad idea either, as a light that is blinking gets more attention than a light that is steady, and a light that is moving with my head would get more attention than one that is (relatively) stationary on the bike. But I'm not sure how my helmet would accommodate a light; I could probably rig something up with a velcro strap through the vents, and I'd also have to use a rubberband, because just using the "pocket clip" to hang this light on the strap on the back of my seatbag does not work, it falls out with the slightest bounce.

Not $5, more like $20 but I'm posting here in case this is an issue with other Cateye lights - even though the function is very good, the red bar on top is the on/off button and mine fell off after about a month so now I have to carry a little piece of wire to poke in the hole where the bar used to be. Also had to zip tie it to my rack but that worked pretty well.

I can't speak to the method using that material, but here's a tip I picked up from here using an old inner tube as the material.

If you just wrap the tube around, you'll notice that your mounting bracket won't stay put. Then someone pointed out the error of my ways- I was missing a 'friction agent'. Tape one end of the tube to the rack leg/strut, wrap the tube around itself a few times, then tape it to itself.

I recently bought a PDW Radbot which came with a mount for the rear rack.

One that I had bought a year ago didn't have one. Not sure if they are new or maybe someone took it out of the box of the first one I bought? It wasn't too big of a deal because the guy at my LBS made one for me out of the post mount for $5. It looked great and worked perfectly.. I'd post a picture but that bike was stolen

If there are two holes on the metal tab, wouldn't you put a strap through the holes, and tide the strap to your blinkie? If it's loosely strapped, stuff whatever you have into the gap.

Yeah, that's another thing to try; it's the 'filling the gap' that would be tricky. But I'm still planning to go with the metal tape concept from rumrunn6 above. Problem is I just moved, and I don't know which box it's in. (And boxes full of garage stuff will of course be the last to deal with...)

"drill a hole into the plastic clip/sleeve I already have, and bolt it onto the rackmount (minus the screw-on stay-clamp)."
thats what i did for my beater/commuter

Drink a bottle of wine and use the cork stopper as a filler

OK, so I jimmied up a solution using metal tape similar to the above, after a few months it broke off, so I fashioned a second one, a little sturdier, that one lasted about a year before it broke off mid-commute, causing me to lose my blinkie forever -- with a pair of rechargeable AAA!

So warning to anybody that tries metal tape for this; check your tape periodically (monthly?) for cracks and replace. Make a handful at once, so replacing is quick. The problem (for me) is that with the light sticking out like that, it gets bumped/bent almost every time I flip the bike upside down for maintenance. Eventually, the repeated flexing causes breakage.

I have a seatpost flasher... so I actually have a trunk bag that has a little strap in the back to hold a flasher. what I did was to take the flasher off of the seatpost holder, stick the seatpost holder through the tab so that the part that holds the flasher was above the strap and the seatpost attachment was below the tab... attach the flasher. it works fine.

another alternative: take 1/2 inch pvc pipe (size so that there will be enough to use as a "seatpost" screw the pvc pipe onto the back tab and voila, you have a seatpost that you can use your flasher for...

no tape involved. sorry I haven't replied till now. that's just metal strapping from Home Depot. (I love that stuff) I got lucky I guess cuz this invention is still going strong and doesn't seem to move. I guess it's a function of how short the mount is and the amount of friction caused by the squeezing. I also made weird mount for my left drop bar. man that thing is weird but also works reliably (several years now). That one bounces a little but for the most part it stays where I put it. Here's a pic of that weird setup.

This is what I do. I did not notice that strap for a few years, until I noted the strap promoted as a feature of a different trunk bag.

I wrap a rubber band around the end of the clip below the strap to keep the light form bouncing off the strap. I also do wear a blinkie on my helmet.

That's what I mean, I call that stuff "metal tape"

I have a strap at the back of my seatbag as well, but if I put the light there it would sag downwards, directing the red light toward the top of my rack, rather than towards the eyesballs of cagers.

For now I am using another light I had around in the garage; the seatpost attachment is more of a plastic strap than a clamp, so I have tightened it around the back of the rack.

well actually, that can happen but I am guessing you didn't really try.... what you have to do is use the adjustment for the angle so that it keeps it upright...

Put this one on some pvc pipe I screwed to the rack