I made a screen lid with 2" wide birch planks, door screen and a staple gun with shallow staples. I set it in the lip of the tank and use those giant black paper clips to hold it on. I used hinges as well so that i wouldn't have to take the whole thing off to open it. I have made 6 lids now and this one by far works best and was the easiest to build.
I'll grab some pics later if I remember. There are definately more than a few ways you could do it, get creative.