On 27 Feb, 01:38, J. Clarke wrote:

Glass, then paint.

Anything else is a heat hazard. Acrylic and most common plastics
certainly are.

That depends on the distance from the bulbs and their wattage.

Distance is fixed by design and the brightness may need to be
unrealistically high so as to still be visible on stage. You'll also
have to OK anything you use with the fire marshal, and I doubt if
they'll accept any plastic. It might be OK, but it's not _provably_
OK.

In particular, lighting gels are well-known as a theatrical fire
hazard, if used close to a bare bulb. They're flexible and can easily
be distorted so as to end up closer than you expected.