Id it's a regular aquarium (rectangle) then you could get a piece of plexiglass cut to the size if the aquarium. But make sure you leave about 3 -4 inches of space in the back for filters and gas exchange
If you're just looking for a lid, I'm partial to glass canopies. If you're going to build a big wooden canopy, paint the inside white and give it several coats to seal the wood well. The white will reflect more light down into the tank.