It helps a lot to mount a little 12 vdc fan in DIY hoods, to force ventilate the light enclosure area. I found that moisture collected under my hood when I didn't have the fan running, causing the wood to warp. The fan eliminates much of the heating of the water by the lights too.
Shellac is not a good finish where moisture is going to be around. Shellac turns white when it gets wet and stays white. Polyurethane varnish is a good choice, and you can apply it over a shellac based primer if you wish, but it really doesn't need a primer. Use at least 4 coats of the urethane. Tung oil is another bad finish to use if moisture will be present - it does not build up a coat on the wood, so the finish ends up being very, very thin.