-- Last Updated: Oct-17-12 11:08 PM EST --
Getting a good non-OEM seat in a kayak is a non-trivial exercise. Having built a handful of kayaks, I tend to prefer carved foam. Doing it yourself can be satisfying, but may also be frustrating and expensive (like, if you mess up your slab of 4-inch thick minicel, which is expensive to begin with). Another possibility is to either get a full custom foam seat from Redfish kayaks, or get a bottom piece with butt-print pre-carved in it, from Redfish, then, you'd shape the block to the hull outlines, yourself. Probably use Velcro to hold it in place. Oh, the WS seat pan does provide a bit of structure, but, I doubt if it's enough to be critical.
http://www.redfishkayak.com/seats.htm
http://www.redfishkayak.com/foam.htm