The big issue with humidity that people don't seem to understand is simple humid air rises. Screen tops loose heat and humidity. I would NOT suggest blocking off more than 1/2 max 2/3 of the top. This practice is well documented to be a strong promoting factor with RI. I also would suggest a coco coir (eco earth) substrate it holds a huge amount of water. You can squeeze it out with your hand. I would never suggest the top surface be wet damp but not wet.

I found this to work very well to increase humidity.

place a glass pan (baking dish) or plant tray on the bottom of the enclosure on the cool side. Add water to the dish not enough so it spills but enough that it has at least 1/2 full. Bank the coco substrate so it is very deep in and over the dish (2-3 inches). the top layer will stay dryer if not add some more dry to the top. I found this held so much humidity that it would take 4 or 5 days to drop below 50% Every now and again you have to top up the dish just poke a hole with your finger down and pour in a bunch of water (slowly) until the dish is full cover over the hole.