Plexi, with a hole cut on one side, is a good cover...but I find foil works just as well and is much cheaper. :D Easier to install and fit as well.
Snakes don't need a ton of ventilation, so the one hole on one side is just fine. Remember that no enclosure is totally sealed, so air gets in from the edges of the lid as well. :) That, plus the hole over the screen, allows good circulation while still keeping temps and humidity where you want them.