Just read your first lighting post, and I use a 6% Arcadia for UVB. Very good penetration throughout the enclosure due to the reflector on the fixture. Bonus with these is that they should last up to a year as opposed to the standard UVB's that need to be swapped out every 6 months. As for the positioning, definitely on top for chams. They need places with shade to escape the heat and feel more secure if they want to. I would suggest a UVB tube bulb as well as a 6500k for live plants. The better the reflector in the fixture, the more effective it will be. As for the screen, do not use any of the tight mesh insect screen and you should be O.K. I use a vinyl coated pet screen on the sides of my enclosure, but an aluminum screen on the top so the basking bulbs do not melt the coating and enough light gets through.