Originally Posted by
Bekologist
Its too bad Jannd -or any mainstream bike accessory manufacturer to my knowledge -doesn't offer a rear triangle bag.
its quite a handy place to put a tool bag on a long chainstay'd bicycle. I believe they were available in the 1970's
Indeed they were. I lost the one I bought back then, but when I purchased a used mid-'70s Trek a couple of years ago it came with one. It could be bigger, but comfortably holds a multi-tool, two tubes, levers and gloves.
It would be pretty easy to DIY one. Four velcro straps (two for the seat tube and one each for the seat stays), a zipper and whatever material you choose. Since its location is clear of your legs it doesn't even matter if it lumps out a bit.