A cover will not deflect uv
. You need special glass for that to happen. If you use fibers glass, over time It will become less efficient from scratched.
Your lights his to close from the water, i have It at min 4 inched for 2 reason, less heat and splashes.
I don't know how well is you light made, but simply say. Electronic and humidity will make some corrosion and eventually brake down.
One more thing, why would you what to clean you light every day? Fix It so you don't have to do It anymore
Edit : if you add a cover, you can remove your cover from the lights.
It normal for aquarium lid light to have some degree of splash and humidity