Preferably something reflective, white plastic, etc. If you got a good hood then do what you like. Its not gonna kill you if you keep it wood. If you're like me and you want it the best then you'll cover it with something.
so white plastic is fine for the roof the heat wont melt it or catch fire, i'm using a 400 watt HPS with cheapo alumabright hood... in a couple of months i am upgrading to a vented hood.