Yeah, that looks like a good size. Didn't realize you had no grab rail. You probably tried it already, but is there any way you could run the straps under the seat and still be able to quickly release the bag from them?
Just received an Axio Tail Pack that I would recommend. It is a nice size for the stock Bonneville (SE) seat. The Axio blue is a fairly nice match for the SE blue. Will hold a rainsuit & a tire repair kit. Size is (US) 14"w X 14Lx7h..sorry no pics. This is a Hard Pack.
damn, if I'd seen this before I probably would've picked this one up. It'll go in the memory banks if I ever find the little bag i got is insufficient.