You could carve a master plug from balsawood then seal it with thin cyano, or use basswood for the plug, then vacuform sheet styrene over it. I have built several vacu-formers of different sizes. I made one small one for doing small items from an old small electric hot plate some masonite for the vacu chamber and use a vacume cleaner to creat the vacume for drawing the softened styrene down over the plug. I will dig it out and take some pics of the vacuformer. For larger vacu-formers I have made the chamber out of old speaker cabinets and use the oven on broil to soften the styrene then with the styrene softened i just turn on the vacume cleaner then remove from the oven