If you want to cover your tank top, use eggcrate available at Home Depot and the like. This will allow for air movement, easy access for adding rock, corals and animals, as well as keeping those "jumping" fish from performing Hari-Kari onto your Oriental rug.
Since I have never use this stuff I am curious how it would be under MH lights! Would it be to hot? Maybe others will add info to this discussion!
in my previous tank i hung fiberglass mosquito netting over the opening on the back of the canopy. i worked for many years. i would occasionally see my goby get wigged out and leave the water, and a secound later fall back into the tank, none the worse for wear. this was with a 175w mh's.
What I use to cover the holes at the back of my canopy is needlepoint/latchhook rug fabric. It is a heavy, flexible yet not flimsey plastic screen that is easy to cut to any shape. I just stapled it to the canopy sides and cut out with scissors or x-acto knife for the filter outlets. I get it at the needlepoint craft section at Wallyworld.