I've been thinking for awhile that having a bag on the inside of the frame of the bike would be a good place to store a few small things, such as a camelback bladder, shirt, etc. What I mean is a bag mounted where the yellow triangle is in this pic. For all I know, it's a common thing in bike shops, but I haven't been to one in a few years (unless you count Canadian Tire) and I've never seen anyone with something like it.
Would it be a good idea? Does it exist? If so, where could I get one?