I occasionally commute to work on my road bike and am looking to lose my backpack in favor of a pannier or trunk but i don't want to junk up my bike too much and would love to find an option that can be easily removed for weekend and fitness rides. I've seen racks that attach to the seat post but I'm wary of putting a lot of weight on a carbon fiber post.
There are many small racks from makers like delta, topeak etc. Many weigh less than a pound. These are not seat post style racks. Removing it would probably not affect your performance that much. PLus how sweet would it be to dust someone on you "commuter" bike?
How about getting an inexpensive metal seat post and second saddle, and attach the rack to the substitute seat post? Mark the posts, and then just swap the recreational saddle and post back and forth with your commuting setup. One bolt swaps everything.
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.