tarp or the heavy clear plastic would work- use zip ties for the plastic after you reinforce the holes with ducktape. I'd say use the tarp and get a gromment kit (really cheap!) and do that- then you can use zip ties or wire to attach it. Cut your tarp up into the sizes you need, hammer in your grommets and you're ready to go. Don't do the plexi unless you want to spend a TON of money