I have found that the best "lid" for a saltwater aquarium is aquarium safe egg crate. Egg crate is a plastic grid like sheet that can keep fish in by sitting on top of your aquarium but interferes little with gas exchange/lighting.
Another reason to use egg crate is the price. You can buy on eBay easily (just be sure it's aquarium safe) or from your lfs
. I'd buy white as this will hardly effect your lighting at all.
Finally egg crate is easy to cut and can be cut around lighting or pipe work.
I hope this helps.