Craig, if you have a band saw or the means to cut straight lines in a 3" block of minicell go ahead and make a seat. I've done a few for Chesapeakes starting with cutting two blocks of minicell at an angle so that when they're glued back together it's the same angle as the bottom of the hull panels. That way you've got a full 3" to deal with. IIRC the seat was 18" front to back and 19" side to side. You start defining where the back of the carved part meets the top, about 2-3" from the back defined by an arc. Next is to mark off and cut out where the hip plates go. That ensures the seat can't shift fore/aft. It takes a lot of carving before it's flexible enough to pop into place. The next lines follow from the end of the arc forward. The arc is narrow enough that lines forward from it angle out,,like your legs. But the lines have to be in far enought to not cut out minicell where the hip braces are.
The next set of lines is a small v marking off the inside of thighs. Carve away.
Or order one from Redfish.