Ok, I've been listening to the same question about covering a Ball Python tank for FAR too long, so I'm going to chime in. This is regarding Ball Pythons in aquariums. I've heard people say to "cover the tank partially with..." a wet towel, plastic wrap, foil tape, your mother in laws granny panties.... All decent, but ugly solutions.
Here is a better one! go to your local plastic shop and ask them to cut out a piece of PVC/X for you to the same size as your tank. Then have them cut out a circle that is 2" larger in diameter than your heat lamp (which will leave approx 1 " spacing all around). As well, have them cut (3) 1" holes on the other end. Place this plastic on top of your LOCKED grill lid.
The heat will warm the air on the 1 side of the tank, which will cause cooler/fresher air to be sucked in through the holes on the other end. This will help to keep the tank full of fresh air, while still keeping it sealed off enough to maintain heat and humidity. If you find that you're still having humidity issues then go to the store and buy a reptifogger. Fill is with distilled water and put it on a timer. Run the hose into 1 of the 3 holes. Set the timer to turn on as often as you need to in order to keep your humidity. Maybe once a day for 10 minutes, maybe once hour for 2 minutes.