I built such a seat as a way a cheap way to get my boat on the water and also serve as a prototype for an eventual minicell version. The instructions at kayakfit.com provided all the basic techniques. I could only get my hands on 3/4" material, so used spray adhesive to build layers to the desired thickness. A wire wheel on a drill makes quick work of removing most of the foam (but generates lots of clinging blue "dust"). Heavy grit sand paper and drywall sanding screens were useful for smoothing the finish. The layers of foam made it easier to keep things symmetrical and judge depth as material was removed. The foam is actually coated on both sides with a very thin plastic veneer, which was a pain to deal with while carving, but not a huge problem. Unlike minicell, the blue foam will dent permanently, but once in the boat, it's fairly protected from damage. All in all, it was cheap, quick, and comfortable. Not as good as minicell, but serviceable. It's admittedly ugly, but I can cover with fabric once satisfied with the paddling position. It may end up being hard to justify the time and cost of making a minicell version.