You should not have soaking wet or damp substrate. They don't live in swamps. If your aspen has gotten stinky, throw it all out and start fresh. If you cover most of the screen top you won't lose the humidity so fast; maybe cover all of it except where the heat lamps are. That will still leave plenty of ventilation, because your lamps are not airtight or anything. You can also get a larger water bowl with a bigger surface area and put it right under the lamp. You might also try a different heat source too. You should not have to mist every hour!

But I'm also curious what you're using to measure the humidity, and where it is. What you're describing sounds fairly strange.

One last suggestion that might be worth trying is raising the humidity in the room where the snake is. An easy way to get some cheap, passive humidification is to set boot trays full of water in front of the heat vents (or pans on or under radiators, if that's what you have). In my music room I get a little extra humidificaton that way. It's not a ton, since there's only one vent, but it's better than nothing and it makes the room more comfortable for humans, too.