i feel like every time i go in ski gabber i see a person with less than 10 posts asking about how to build their own box/rail.....not even exaggerating. and whats funnier is that is exactly why i joined newschoolers, to ask about a box. i discovered the search bar though...
usplastics.com sells HDPE in 1/4inch thickness which is what i use...its kinda expensive, but they have a set shipping price of only like 10 bucks, and they have great customer service/ uber fast shipping