If you make a seat that fits you, it'll only fit you, then your buds can't paddle it. I like to share my boats.

Bill H.

Good point.
Also, if you make a 'form-fitting' seat that is molded from your backside when you are 'sitting square to the boat', it may not be very comfortable even for you if you are paddling with leg drive (moving your hips) or moving to face more 'sideways' for things like draw strokes, etc.
Some hard seats which look very plain/generic to me (NDK, Mariner) have turned out to be very comfortable (for me).

That said, I'm not a big fan of the 'wedged-in-the-boat' feeling so I like a bit (not much) of 'slop' in the seat.

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.