I have made many light hoods, and used most, if not all of them with no glass on top of the tank. Not once have I seen a moisture problem in one of those hoods. I have used fans in a few hoods, but not to keep out the moisture, but to evaporate some tank water to help keep down the water temperature. The only moisture effect I have seen was years ago, when I had an 8 tube T8 light on a 120 gallon tank. I put a sheet of aluminum behind the bulbs to help get more light, and when I finally tore down the tank I found some corrosion on the aluminum, but very little. It isn't necessary to water proof a hood, since if you ever dunk it in the water you have electrical problems that waterproofing doesn't solve. So, you have to avoid ever dunking it anyway. All plywood that is commonly available uses exterior grade adhesives anyway, so the days of plywood delaminating due to moisture are long gone.