No pics, yet, but here is what I am doing.
Closed cell foam from here
http://www.austinkayak.com/catalog_search.php?keyword=Closed%20Cell%20foam. Best prices I could find and several choices for thickness. It may be just a touch short since it's 24", but you can buy two pieces for less than anywhere else I found it.
Pre-pleated marine vinyl for the top. Lots of choices here
http://greatlakesskipper.com/category-1/14_94_917-boat-upholstery-pleated.html
Some leather grain look marine vinyl for the sides. I bought some cheap stuff off of amazon, but there are lot of choices if you search around the greatlakesskipper site I linked above.
Some hidem for the transition around the top from the pleated to the non-pleated.
Unfortunately, I don't have a sewing machine. But my mom does

!
We'll see how it turns out.
I found a shelf from a metal book shelf that I'm making my seat pan out of. I have a rough one that I'm using to make a pattern for the final piece. I bent the rear area with a piece of drain pipe to match the rise of the rear frame loop. Going to rivet the seat cover through the bottom of the seat pan.
I'm probably going to do the rear fender similar to EWAN's but haven't decided on it, yet.