I use Styrene Lighting Panel for my tank lids. I prefer the "Cracked White Ice" style by PLASKOLITE. It comes in 2' x 4' sheets and is readily available at Home Depot or your local hardware store. It cuts easily with a razor knife and straight edge and one sheet will easily make covers for 6-7 ten gallon tanks or about twice that in 5's. It's also very economical. One sheet costs less than 5 bucks. That's less than a dollar a lid!
I also add a few extra features. First, I apply small adhesive plastic dots just inside the bottom corners to prevent the lid from sliding. Second, I silicone a leftover plastic bottle caps on one end for a handle, or you could use a marble and Presto!! Instant lid. I have been using styrene lighting panels for 3 or so years with great success and I haven't had one sag into my tanks yet.
One Of Ken’s breeding tanks. This is a 7 liter (2 gal.) critter
keeper with a DeBruyn style Wet/Dry filter and a Styrene lid. The lid has an
inverted bottle cap for a handle, and some adhesive plastic dot that usually hold up glass
table tops. The dots, which look like short, dark crescent marks near the corners,
hold the lid stationary when it is on the top of the critter keeper.
Ken used styrene lids on almost all of his tanks. They keep the fish from
becoming carpet crunchies, but also is light enough to make lifting a cinch. Plus
these lids prevent eveporation. However the DeBruyn style filters creat a ton of
evaporation. This will necessitate addition of RO or DI water to correct the loss.
Ken has over 200 tanks in this room alone (other side not shown.)