Got one of the Wally-world Bell bags. I think it was $26 +sales tax. I use it to carry bungie cords, a mesh & vinyl bag (for store stops) and some other small things.
For size estimate, it will hold four 12 oz Soda-pop / beer cans.
On several occasions I oops's and forgot to strap it shut. It is stiff enough that the flap stayed closed.
Only issue is that I had to put some black duct tape on the fender (my bike is black) because it was touching down on the paint and leaving marrs. I guess from fast stops.
http://www.users.fast.net/~tommack/Tmack1-LS650.jpg