I wouldn't worry about a screen blocking UV rays unless the screen is for some reason very heavy. Standard aluminum screen or hardware wire mesh is used on the tops of 95% of chameleon cages, and I think everyone takes that into consideration when saying that you're supposed to be within 12 inches of the bulb or whatever. I made a cage once that had a solid plywood top with pieces of it cut out for screen, and where the florescent light sat on top of the plywood, it was open, so the chameleon could directly be under the light (no screen in between) but it became more of a pain because if the light moved a little bit then there was a gap where insects could get out and even without that gap, it created an easy way to escape for insects and chameleons if they moved it a little, and I would think a big cham could. Kinda hard to explain what I mean. Just keep a basking spot close to the bulb and don't worry about the sceen blocking a little bit of it.
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Tyler Stewart

Las Vegas NV

1.2 Nosy Be Panthers

2.2 Sambava Panthers

1.0 Tamatave Panther

1.0 Nosy Be X Unknown Cross

1.1 Veiled Chameleons

0.0.2 CBB Desert Tortoises

0.0.1 Sulcatta Tortoise

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