Please feel free to learn from the errors of a Tank Lid first build ...
I did have the tank up and running for about 3 months before I put the lid on & I figure I'm only losing about 1/2 the water due to evaporation compared to Open top.
This is the lid on my 45gal Display. Inside I've got a Coralife with 2 x 96w CF - there is a bit of a heat issue that I'm working on. The light has fans which I've vented, but there is still a heat issue - probably nothing like MH, though.
If you build a tank cover, take my advice and unless you have a serious space issue don't attach the lights to the hinged lid. Great access, lots of room to get into the tank, but its really hard to see with all that light in your eyes at feeding time
With the Lid closed:
With the Lid open:
