This instructable outlines the steps required to build an inexpensive wooden recumbent trike seat from 1/8" plywood. Any thin sheet of wood could be used but I found 3-ply birch plywood bent quite well. I purchased the plywood from my local Rockler Woodworking store. The finished seat is comfortable and very sturdy. I am 6' 1" and this seat fits me very well. If you are much taller you might want to make the seat a little longer.
If you want to see how I build the bamboo trike frame, check out my instructable:
Step 1: Making a template
The first step is to make a template of the curved seat. I downloaded a PDF printout of this curved seat at http://ia700209.us.archive.org/9/items/RibSeat/Rib.PDF. I've included a copy of the PDF below. The seat outline prints on four sheets of paper and can then be taped together on your wood. I used a 1/4" plywood scrap for making the template. Transfer the outline to the wood and cut it out on your bandsaw or with a jigsaw. Make the cut just a bit bigger than the outline and sand it to the line using a sanding drum on a drill press (pic 2 & 3).
In the first photo, you can see the final shape of the template after cutting and sanding. Take your time making the template since it will be used to make all the other pieces on the router table with a flush trim bit.