They typically make the tops out of HDPE which can be found online I believe, yet it is very very expensive for the average consumer. PVC is the cheap alternative for kids now a days
i think its called lexan. i heard its like 400$ for a 4x8 sheet but im not sure about that. black ice also works its a material they use to line the bins for stone at quarrys. thats what im using for my box. hope that helps.