How handy are you?
Some years ago I read that people were putting together large soda bottles to make water-resitant panniers of a sort. Back then, there was some diameter differences between brands, so different bottles could be cut and fitted together. With some reative cutting and taping, you mght come up with some tubes and with some webbing and buckles come up with something that can work for you.
I ust ordered a folding bicycle (20" wheels) that should be able to fit behind my driver's seat in my little Chevy HHR.I will probably make something like these for my bike. Reasonably cheap if they work, replaceable if I damage them. Hard to beat.