Good post! I do a similar thing with the light panels they make for the ceiling mounts. I found it to be cheaper and it does a good job dispersing the light i have for the tank. I cut it down to size to fit a 55 gallon screen top. I also cut out a circle for the heat bulb I use. I will say I bet plexi might be easier to work with because the plastic I used was extremely brittle and took a bit of time. To be honest, if a ball python has more than 1/2 of any side of the tank open with only screen, and it isn't in a room with a correct humidity (most houses don't get to 60 because of central air) then it is probably not humid enough. Sure you have a giant water bowl, but humidity is based on atmosphere saturation. All a big water bowl will do is make the area an inch around the bowl a bit more humid. But underneath the hot hide won't be affected if the water can't stay to saturation the air.

An even cheaper way that I used for my pacman frog is to use double sided tape and those clear plastic window covers meant for winter to keep in heat. With my pacman I gave it full coverage but with a ball maybe leave the and inch open on the ends or maybe don't tape the ends down.