If your not using the light fixture for heat, the you don't really need it. There are a lot of different things you can use to cover the screen top. I keep my boa in a glass tank with a screen lid and I cover pretty much the entire thing with a damp towel. My humidity has been great and I get perfect sheds. I still mist once a day and make sure the towel stays damp. You pretty much need to always keep the top covered to keep humidity in.
Tubs are much better at keeping humidity though.