mass-manufacture plastic
nkurz: I've been doing some home molding with High Impact Polystyrene (HIPS) and with clear PETG.  The polystyrene is opaque, white, strong, and particularly easy to work with.  It's also cheap (about $50 for a 1/8" 4'x8' sheet).Vacuum molding at home with an oven and a vacuum cleaner can produce surprisingly good results.  Searching the web for "vacuum molding" or "vacuum forming" should turn up some decent tutorials.TAPPlastics.com is pretty good for small quantities (especially if you are near one of their stores), but others (like Interstate Plastics) have better prices on full sheets.  TAP has HIPS at their store near me, although I don't see it listed on their web site.