I thought about that but the plastic will still get hot. Metal is the only way to have the bulb that close to the sides. when I was testing I placed the plastic inside a chrome cover and the chrome cover got hotter than 120 deg (I could not pick it up with my fingers) just from being near the bulb. Think easy bake oven 60 watts can cook a cake!
I believe LEDs would be acceptable BUT since someone down the road could put an incandescent in it, I would be remiss to make it out of plastic. as for the paint it might move some of the radiant heat BUT my testing was done in open air. The assy is actually closed with the lens and the gasket so all heat loss must go through the housing. Metal is the only way to do it fail safe.