I have used really large underseat bag, I looked at nashbar and performance and didn't seen anything that looked that large......The bag i have is about 18-20 " long about 6" wide and maybe 8 deep. It has a plastic piece in/on the bottom that keeps it from sagging too much. velcro loops on seat rails and set post. It also has a strap that goes around so you can attache stuff underneath. I could put a change of clothes in it and attach shoes to the bottom.

you might want to look for somthing like this