Safe to assume that this is an aquarium that you plan on using as a reptile enclosure? If there is an inside lip at the top of the aquarium, you can make a screen to fit into the slot very easily. Got to your local lowes/home depot, go to the windows section, look for the rolls of screen, and next to the rolls of screen they have the metal outside frame work for the screen to sit into, grab some appropriate size rubber to secure the screen to the frame use the screen roller to secure said screen to the frame and voilà!! I can take some pics of a screen top I made for a corner tank my buddy is going to use to house his rat snake in, turned out great!!