Sometimes I need just a bit more storage on longer rides. Here is a small and light rear rack/bag that fills the gap between an under-saddle bag and a full rack:

https://www.nashbar.com/bikes/Product...200545__203224

There are a couple of different sizes. Quick to mount or remove when not needed.

My saddlebag is way too full for even a thin jacket. I carry spare tube, patch kit, 3-oz bottle chamois lube, wet wipes, spare blinkie or battery, emergency clif bar/shot blox, CO2, levers, wallet/ID/insurance, keys.
My jersey pockets are busy with 5-oz flask of sports gel, cellphone (running cyclemeter), clear safety glasses (when ride continues after sunset), possibly a park gate pass or work badge to get through gate. Only things that I want fast access to on the roll.
The windbreaker stuffs into its own pocket and has an elastic waist belt with a metal buckle. I usually use a velcro tie on the buckles to fasten it to the bike saddle over the saddlebag.
Alternately, I'll use a thin drawstring backpack. If I'm bringing along a windbreaker, temperatures will be cool enough that having an ultralight pack on my back won't get too hot and I'll probably have tights or other apparel I'll want to ditch or swap out for lighter versions.

I gave up water bottles for a Hydration pack. It has a storage area for small stuff and a webbing pouch that pulls out for larger items like a jacket.

Rolled up and stuffed into the jersey pocket. This way, when it starts raining, you can pull it out and put it on. While you're riding, of course. Just make sure you always keep the collar of the jacket at shoulder level so it doesn't get sucked into the rear wheel. And having a bike that tracks straight is also good.

For Paris-Brest-Paris, though, I didn't have room in the jersey pocket for the rain jacket (the vest was already in there). So assuming that, like me, you don't like to put racks and stuff on your bike, I just lashed it to the seat bag (with the velcro strap that was supposed to hold the timing chip, since I had the timing chip attached to the velcro strap on my cycling shoe).

But if your bike has racks and stuff, no problem. You don't even have to learn to ride no-hands!

Luis

It's either on my body or in a pannier.

Has the OP checked the bike department of his local x-mart? Bell has a small handlebar bag.

If you're handy with a needle and thread or know someone who is, you can do what a number of people in my bike club did. They bought thin nylon stuff sacks at KMart or WallMart. They are found in the camping section. The bags usually come in a three pack so choose the size you need. They then sewed two six inch+/- pieces of velcro, perpendicusar to the length of the bag, to the sides. Don't sew the velcro on at the end but in the middle. Jacket goes in the bag, bag is closed with the enclosed line lock and the velco wraps around the handlebars.

I'm thinking of going for one of these top tube bags. Does anyone know if it will interfere with the brake and shifter cables on that are on top of the top tube on my Trek 7.5?

Spud

I think if I saw my wife doing that (even as well secured as you described), I'd just shove it in my own jersey pocket.. but then again I never bring a jacket (the one's I've tried flap in the wind too much) and when I ride with my wife it's usually slow paced enough I'll bring a backpack and carry extra food/water/etc..

There aren't that many days that we need a jacket here in Florida, especially this year. I usually carry some long shoe laces in my saddle bag that have several roles. When I do wear a light jacket or sweatshirt and it starts to warm up, I roll the jacket up and tie it to the saddle bag with the shoe laces. Works really well and you don't have to give up a jersey pocket or have a large bulged item against your back.

Shouldn't be a problem unless your stem is completely slammed down. The straps can go between the tube and cables. I occasionally use a Bento Box like this on my road bike: