sells narrow, plastic strips you can cut to fit to cover the areas behind the lighting, but as you no doubt know, some evaporation is normal. Could be considerable if you have large tanks. My tank lights are on 12 hours a day. I have a lot of tropical plants and they're used to long hours of daylight.
I have several 55 Gs and the evaporation keeps the humidity in the house quite comfortable. Really keeps down the static electricity during the winter months. I dose liquid ferts mixed with a little of my water change water in the evenings after the tank lights go out, so my tanks are always topped off.
Just a natural thing that keeps a little new water in the tanks between large water changes.