I made a mold that I've used for 6 boats so far and am now doing again.

In a pile of sand, sculpt a seat. I made a tractor type with an upward slant. Carve it to fit your botton like a glove (keep it symetrical) Line it with thin plastic bag material and fill the indentation with plaster of paris. Fine tune the mold with wallboard compound and sanding etc. - then coat it with epoxy. I mounted mine on a small 6" stand which makes it easier to lay up cloth. It's easy.

I don't use release agents since I had my layups stick on a few occassions and took a chunk of mold out so now I lay pieces of seran wrap over it for releasing. I first coat the mold with auto wax which makes the seran wrap stick well.

It takes about 6 layers of 6oz cloth to get the desired stiffness. Pull the glass off as soon as it toughens up. Dont abondon it for days or you may need a jack hammer. Later I sand and coat/fill any imperfections on the seat side. I also add graphite powder or copier toner to the epoxy to make it black.