Glass Top?

Measure the inside of the rim, subtract 1/16" off the short side and 1.5" off the long side. Take these dimensions to your local glass shop and get a piece of tempered double strength glass cut to size (thicker on larger tanks). The gap at one end is for your cords, hoses, feeding, etc. I have even used a HOB with this set up.

Oh, and have them cut off the opposite front corner (1.5 x 1.5) for a second feeding hole and to make it easier to remove the lid for maintenance.

This setup does impose some restriction on which filters can be used (can't be any wider than the short side of the tank), but it maximizes light transmission, minimizes evaporation, and is inexpensive.
 
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