I made one out of plastic window replacement plastic (home depot).

I cut out two over size pieces and sanded the surface with 100 grit and epoxied them together with a piece of 6 oz cloth in between. later I covered both outside surfaces with cloth as well and filled the weave. Unbelievably strong.

When you glass the outside do it in one shot by sandwiching them between scrap plywood and wax paper. Doing it that way the glass comes out with the weave filled.