I made a butt casting seat years back. Simple. You make a little sand box and hollow out an area to sit in - not too deep. Then you pile up the sand a bit around the outside of your shape. Dampen the sand with the mist spray from the hose so it holds shape. Once you get up you visually sculpt it as symmetrical as possible with that lump between the thighs. That's an important shape (like a tractor seat) because it keeps you from sliding from side to side. When you're satisfied, you lay on some garbage bag thin plastic and smooth it out as best as possible. Then fill it with plaster of paris. It will harden in a short hour. You can then fine tune the mold with speckling compound filling any dents or imperfections. I coated my mold with a layer of 3 oz glass cloth and filled the weave and sanded it as smooth as possible.

When I do a seat, I coat it with Vaseline and use garbage bag plastic as a release. I tried mold release but had a awful time getting the seat to release so I just use the plastic bag material. You may get a wrinkle in the glass seat but I fair it out with epoxy putty. Usually a minimum of 6 layers of 6 oz cloth or equivalent will give you a strong thin rigid seat. In the beginning it doesn't seem strong enough but in 4 - 5 days it gets super stiff.

I used that mold for many boats then I made another one off my brother's Pintail glass seat which is the tractor style seat. when you have a glass set that fits you well, believe me, you won't need any additional padding. My mold is mounted up on some wood about a foot high so the glass can hang over the sides which I trim off later with a jig saw.