: Which "commercially available seat" did you use for a
: mold?

Bill, I used a sculpted seat blank from Joe Greenley at Redfish Kayaks. Most fiberglass kayaks have a fairly good seat to mold from.
http://www.redfishkayak.com/foam.htm

By the way, one of the great things about Greenley's seats is that the sides are high enough that I don't need to build hip plates in my kayaks anymore. Even rolling, those seats are enough for me to stay firmly in place.

John