cage
Just let the soil in the plants help with humidity. I don't think substrate is worth the risk. All my chams do fine and I can never get the humidity up too high. Unfortunately, the air conditioning in your house is designed to remove humidity so it is always a struggle to keep humidity high - most people use misters (like Mist King - I have two) hourly but your fogger will help. 50% is not that unusual Some people, me included, use plastic sheeting on the outside of their cage to hold some humidity. I line three sides of my cages with black plastic on the outside leaving the front uncovered. I use a bit thicker plastic but you could cut a heavy trash bag too. Allows air circulation, holds some humidity and keeps mist or water from getting out. I leave the plastic about an inch or two short in the back to allow for a little more cool air to draw in from the bottom. I still think shrinking down your cage is the right approach. It just allows you to control the environment a little better and be sure that your cham is eating. True that the chams in the wild don't live in small areas but a lot of young chams die in the wild which is why chams lay a ton of eggs. Not hard to shrink things down for a few months so why not do it.