Can also get UHMW from them. It's not cheap though. Most people just use 1" pvc tubes across their box which is cheap and works well for backyards.
I think you might be able to get pvc sheeting at home depot lowes. I know you can get lexan their as well but that's not cheap, the pvc is.
Another advantage to pvc tubes is that you have a round coping automatically. IF you run sheeting you're going to want to throw on coping. And if you spend money on hdpe you might as well use steel for this. You'd have a couple options, toss some angle iron on the sides flush with the top sheet, or frame out your box with an offset on the sideso you can throw a steel tube in there like 1.5". Could also just stick the tube on the outside with a 2x4 below it and bolt it into the box and the 2x4 below will keep it rock solid.
That's all guessing you're making it out of wood though. If you're building it out of steal definitely use a good topsheet.