Custom lid tips?

My tanks lid has been damaged one too many times by my cat, so I have opted for a new lid that my dad will help me build. He knows what he's doing building wise, but does anyone have any tips on how it should be made?
Tank is 36" x 18" x 18"

I'm assuming you want your dad to build something strong but inexpensive? You can find locking lids quite easily by doing an internet search. But I built many lids when I raised reptiles for my son's pet shop. I bought aluminum screen frames, corner pieces, retaining splines, and aluminum screen from Home Depot. I made my own lid locks, but they are available from reptile supply companies. After I built the lids I sealed them with silicone sealer so the screens can't come loose in the frames. Your cat can walk all over the screen and do no damage. Having said all that you can make a wood frame and attach screen to it much cheaper.

You can also make a reinforced cover to go over your original screen. You can make a fancy one with wood, or a fast and easy one by bending that stiff quarter inch welded wire hardware cloth to fit. Something that just lifts on and off. And your original screen remains safe and secure.

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.