>>My main question is for the screening, is there heavier duty screening then window screening to use....something rigid?
One thing I've seen used is that plastic, egg-crate stuff made for covering 4' fluorescent light fixtures. Very easy and safe for a chameleon to climb on.
Not the cheapest stuff and it's easy to crack, but it's what I'd try if I decided to keep chameleons again.
Another option I've seen is to go ahead and use screen but then build up an interior lattice made from thin bamboo dowls. Also a very nice effect.
Neither of these examples is insect proof without an exterior layer of screen.
>>Also how do you finish the wood...any special vanish that is non toxic?
I'm skeptical of the recommendation for Polycrylic or other water based finishes. It's likely you chameleon will drink formed water droplets off of these surfaces on a regular basis. A water based product will partially dissolve into the water meaning your lizard will be drinking a dilute solution of polyurethane much of the time. It also means frequent refinishing of the wooden portions of the cage.
I'm not sure what I'd use. I imagine I'd probably skip wood and try to work with square aluminum tubing. If I really wanted wood I'd probably use a two part epoxy. So little would be needed on a simple frame that the cost would not matter.
Good luck.
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Current snakes:
1.1 Gonyosoma oxycephala - (Silver/Yellow)
3.4 Gonyosoma oxycephala - (Green)
2.1 Gonyosoma janseni - (Black)