just use a large piece of sheet glass (like the type in a window) and make a box out of it. It wont break and cut your legs the fuck up when you go on it. Just trust me iv tryed it it works real good
we used a wood frame and then a local construction site gave us some left over 4 inch pvc. its harder than normal pvc so it wont chip. it also slides really well
that seems kinda sketch 1) i dont think it would slide to well 2) its kinda thin and i think it would break really easily 3) it would b really hard to attach to a box