Evaperation!!!

[Coasting]

I use an acrylic lid since I have tons of acrylic, otherwise I would have used glass. This was to help keep pet hair, dust and fishing cats out, and fish in. No algae on it.... And it doesn't get salt "creep". Since the water side stays humid and moist, its just usually wet. There are multiple holes drilled through for some ventilation and pipes/cords. The top is where I get dried salt from when I remove a section of acrylic and put it on top of another while I fish into my tank. I clean them... every few months.... maybe.... Their reasonably dusty and salty on top, light doesn't seem to have any issues going through it, and my corals grow just fine with lights at 30%. No issues with temperature, and our house gets scorching hot in the summer during the day, with a severe lack of AC (85+... 90ishh... then I stop looking). Tank is usually kept around 78, think the highest the tank went in the summer was 82 or so... I had more issues with it getting too hot when I used egg crate on my small tank in the summer.
So if you actually cleaned yours on a reasonable basis, I don't see a big issue.
Works peachy keen for me, doesn't mean anyone will agree with it or if itll work for your situation.