Glass or Perspex are the only real options. You can cut either to size and shape although Perspex is easier to cut curves in.
If you cut a corner off the front and make a triangle with sides about 6 inches long, you can stick a couple of bits of glass/ Perspex on the side where it doesn't sit on the support strips and glue a handle on it so you can lift the corner up to feed the fish.
Covers get dust on them and sometimes algae. The dust and algae will reduce light but just clean them each week and you're good to go.
If you get glass, make sure it is 4, 5 or 6mm thick. Thinner glass (3mm) is what most shops sell, but it chips and is crap. For small tanks I use 4mm thick coverglass, for bigger tanks I usually go 5 or 6mm.
Thin Perspex will sag in the middle over time. If you get thicker Perspex it is less likely too, and if you have smaller covers (1 foot wide x the width of the tank) they sag less, and you can glue strips onto them to help reduce sagging.
eg: if the tank is 4 foot long x 14 inches wide x 18 inches high, you make the covers about 10-12 inches x 14 inches.