Bill's advice to 'steal' a shape from a seat you like makes a lot of sense to me -it's what I've done. It's easiest if you are copying a hard seat, IMO.... not the types with glued-on foam butt pads, etc.

Hi John,
The most comfortable seat I have ever used, in motion, was a iron seat on a Massey Fergusson 35. No cushioning beyond the curved strap/spring at the front of the seat. Twould make a fine mold for a kayak seat, except that the seat pan has holes for ventilation. And for letting the rain drain off the seat. :)

I haven't come across any CAD files for kayak seats, though I haven't really been looking. I'd likely use the sandbox and wiggle method for seat molding. The sand will conform to your butt shape and the wiggle will give you that "slop" you'd prefer. Less wiggle gives a closer fit.
Mind you, this doesn't help Craig too much.

Mike Savage
South West Cork