i personally would avoid wood, because it's going to eventually rot in the water. stick with more plastic-like material such as acrylic, egg crate/light diffuser, PVC pipe, etc... if you use wood, i would use a combo of the plastics to cover it and keep it raised above the water, and if you have a return on your filter that splashes the water's surface, then make sure to try to keep the wood away from that area, too.
if you use wood, the goal is to try to keep it as dry as possible

you could even considering screwing acrylic over the wood if you like the wood look but want to protect it from water