This might be an options for some:

http://www.teamestrogen.com/prodDT_2300.html

I didn't try it because my seatpost is carbon.

Topeak and REI/Novarro have seat bags that clamp onto the saddle rails, and this might also be a solution for some people. They don't fit the carbon rails on a Bontrager Affinity RXL. I might have been able to get the REI bag to work by substituting longer screws for the ones that came with the bag, but decided not mess with it because it was a bit wider at the seatpost end than what I was looking for.