I know this is a very late response.. but I found the thread by searching and wanted future searchers to have this info.
I have a 3gallon JBL picotope with Current USA satellite 36w lighting and wanted to cut down on evaporation, so I found that a 2.5 gallon standard glass lid from petsmart fits PERFECT on the straight edges (left to right) of the picotope top... and also happened to perfectly fit between the legs of the light fixture.
Anyways, I used it on my picotope for a while but since I used the picotope as a reef tank, i had bad saltcreep on the glass and cleaned it regularly. I'm lazy, so it didn't last long, however freshwater will work great. It cut my evap in half I'd say.
Fish on