When I had a tank, I covered the mesh top with aluminum foil and duct tape and that worked really well in keeping the humidity in. I will look to see if I can find the stick to show you how it is done.

I have never had to use moss to help with humidity, but you can also use damp paper towels for a humidity boost. I would just change the paper towels every day or so to keep from having bacteria grow on it.

Allowing your snakes outside loose but supervised I would think depends upon the type of snake. I would allow my ball pythons out, they are a bit slower. But when I had my cornsnake, I would never let it loose. It was way too fast and would have run off in a heartbeat!!

Let me go look for that link for you now about the aluminum foil cover.