While it seems that most people here prefer to take a 3" thick hunk of foam and carve away everything that doesn't look like a seat, there is an alternative method; build the seat from the hull upward using thinner foam. I got this idea from the seat in my Betsie Bay Recluse. You start by gluing a triangular piece of foam to the hull (ofter referred to as a "pleasure pod"), then cover it with a layer of 1/2" foam. The backrest and side padding are fabricated separately in the same manner. The thickest foam I typically use is 1". If I need something thicker, I laminate it with contact cement.

The advantages of this method are that the thinner foam is easier to find, cheaper and much less of it ends up in the trash.