A friendof mine made a mold in the sand. Use damp sand and hollw out a shape that feels comfortable. Keep symetry with the taller hump that goes between your thighs. Cover the indentation with seran wrap pieces and make your plaster casting. Fine tune it with sanding and wall board compound to make the mold. (from my earlier link)

On my first attempt, I let the epoxy-glass lay up completly harden. I worked on bare plaster coated with wax. Many pieces of plaster came off with the seat. This is why I chose to coat with epoxy. I believe that the solvents in epoxy and polyster dessolve through the auto wax. I think true mold release wax won't dissolve - but it's about $8 a can. I heard of people using Pam cooking spray and silcone spray as well. I don't know. The epoxy coating with wax and removial before it fully hardens works OK for me.

Also... this is once case where a polyester lay-up migh be better since polyester is cheaper and hardens stiffer than epoxy. I just can't stand that smell so I use epoxy.