you don't really need a glass top on your tank unless you have jumpers, even then i'd still say get a window screen and put it over. you want gas exchange so leaving the surface open to the air is benificial at keeping the PH consistant and the temps consistant
I used Fiberglas window screen, but thought it cut the light too much. Used a light meter and even the sheerest screen dropped the light considerably. I bought window screen frames and sheer pantyhose (no jokes please) and stretched that into the frame. Little light loss. But the nylon will only last a while, then needs replacing. Very cheap as a pair of large size panty hose can make a lot of screens. Good for lights that don't put out much heat, hot light will melt the nylon.