I wouldn't go with an all-screen top for a snake. It lets too much heat out and provides too much area for nose-rubbing. Snakes just don't need that much ventilation. I would go with a solid wood or plastic sheet top which slides on with channel grooves on the sides. This will make it secure. (Home Depot has a corrugated product that is light and cheap, like cardboard, but made of plastic) You can drill ventilation holes, maybe like 1/8 inch in diameter in a pattern until you feel you have enough air exchange. You could even use pegboard, which comes in wood or plastic. If you're using an overhead light source, you'd have to cut out a way for the light to show through, and screen it so the animal cannot contact the light. Consider LED lighting: very little heat, and very little electricity. You should be heating from below, anyway.