^yeah that's what we used. Polycarbonate.
Close to $80USD at Home Depot for a 3 x 6 sheet or something like that. The stuff is really expensive.
Our's is now broken. So if your going to put the money into it, I have a few suggestions for you.
First of all, ours broke because we didnt have the edges protected. So if you landed hard on one of the edges of the polycarb (lexan) you would start to fracture it little by little. Until, it eventually would crack and break.
So, that's why boxes at resorts have metal bars or square tubing running flush to the box. This prevents you from ever landing on (or even touching) the end parts of the lexan. As long as you stay on the top part you'll be fine.
P.S. The stuff worked AMAZING. So, if your in the mood for dropping $200 on a box, at least do it right so you don't end up dropping tons of money on a broken box.