Is your tank open with MH suspended above the tank?
I use PCs, but maybe this would work for you, too:
You could get some door screen (at Walmart) and cut it to cover the top of your tank...just leave a space open for the lights to penetrate. I cover the back of my tank with it. You can't see it, and it keeps the fish inside.
Another option is to get some acrylic , and have it cut so that you have "borders" on your tank (measure the inside on the ridge) You can get the stuff at Home Depot cheap. It's how I suspend my PCs directly over the tank w/o having to wire the ceiling for lights. It also acts as a barrier to jumping fish.