I use acrylic lids for my 55g tank, 2 pieces that I cut myself with a jigsaw from a 1/4" sheet I bought at Lowes. I cut the pieces 1/2" short intentionally so that I don't need handles. I turn them over once a week. Because they have no handles, flipping them doesn't matter, and though the pieces do warp, they warp one direction slightly, then the other direction slightly each week. It never adds up to more than perhaps 1/8" deflection along the length of the piece in that time.
If you go glass, Home Depot and Lowes sell single strength (3/32") glass only, and only Lowes will cut it for you, at least around here. You'd do better to find a glass shop (the kind of place that makes shower doors and the like) and buy a piece of 1/4" glass, cut to size and with the edges ground. They can also drill holes for a handle of your choosing, if you like, or you can just make the piece sufficiently short that you can get your fingers around it to pick it up when required. Thinner glass would work, and it is surprisingly strong stuff against a distributed load, but it doesn't take much of a rap on the top of the tank to break a piece of double strength (1/8") glass, and single strength is super easy to break.
If you don't want to find a glass shop, you can also order tempered glass cut to size online. It should run you about $20 per piece, iirc.