I bought one from Cabela's for this year:http://www.cabelas.com/product/Cabelas- ... 621362.uts
It's on sale right now for $40 (10-28-13), but I picked mine up on a different sale for $30.
I think it's a keeper. It is light weight, folds down for easy carrying, sturdy, and is nicely padded... plus it has the Cabela's return policy in case my arse eventually crushes it.
On the other hand, if you're looking to build one up, keep it simple like 3200-man said.
I think a short length of 2x4 wrapped in a towel, nailed on top of another 2x4 upright, would suffice. Cut the end to a point and you're off to the races for cheap.
Have you tried an internet search for "make a marsh/swamp/mud seat"?
I'm about as sharp as a marble, but I was still able to do a Google search and found a whole lot of info already posted on this site and other sites.
Good luck, and post pictures of your completed project!