For those of you who really want to build a backyard box to learn and progress, but don't want to shell out a hell of a lot of money for a HDPE topsheet, this is for you.
The material I used is crazy carpets (roll up sleds) the two red ones were 5$ and the small blue one was 2$
-Sort of a pain to install
-Pretty weak (small rips when you slice it with your edge)
This is a little video of how it performs. (Note I am NOT
the best at snowboarding)
Some things I have learned from using the box are that it is better to have some sort of material around the edges as that is where the plastic rips (like the metal the mountains use), and the best way to fasten it on to the wooden top is to use contact cement to glue it first and then fold it over the edge and staple it.
Good luck if you decide to make your own and tips are always welcome.