Don't use Tyvek! Don't use it! Please!
Sorry. We have two gigantic rolls of plain Tyvek that we're always tempted to use, but never do; it is extremely tear resistant, yes, but it's puncture resistance is terrible. I was able to poke through it with my finger.
We're doing the same thing, M, because our robot's shape isn't very lexan-friendly with all sorts of weird angles. We're using truck bedding tarp; you know, the waterproof stuff made to put over truck beds when it rains? It's light as a feather, and you can't get through it with a pin. It's pretty cool
