I have made about 3 different types of canopy, as I recall, and all were made of plywood, small pieces of solid wood, or MDF. I finished all of them with a few coats of acrylic clear coat to make them water resistant. All of them withstood the moisture very well. So, I don't think you need anything more sophisticated than that. When the light is on it dries out the moisture inside the canopy, so moisture doesn't really collect there. And, you almost have to have a cooling fan installed in the canopy to keep the lights from overheating, and to keep the lights from overheating the water. That also helps keep the moisture level way down. If you have any worries about moisture you can run the cooling fan 24 hours a day and that will take care of the moisture.
The best design I have seen for a lighted canopy is one I can't find again, but it had the light fixture suspended with a linkage inside the canopy so when the top was tilted back it pulled the light up and back out of the way, so you could easily do maintenance on the tank with the light still on so you could see what you were doing. When you tilt back half of the canopy you tend to end up either with the light still in the way or with it out of the way but pointed at your face. If someone recognizes this description, post a link to that thread - it was very much worth trying to duplicate.