You know thanks for posting this, I've been pondering on the same thing.
I have a 100 gallon flexarium all set up with plants and stuff, originally anticipating getting an older, or at least not very young baby. But now I've been thinking maybe baby, so I've been wondering about how to deal with setting up a baby cage inside the big cage. My original thought was to just buy a smaller cage and prop it up on a footstool or something to boost it up inside the flexarium. Then I could utilize the existing plants in the big one to retain humidity and provide some privacy around the outside of the smaller baby cage. The outer outer cage would help hold humidity in too.
The problem I saw with that is that then the lights have to work through TWO screens to get to the cham.
Maybe a stronger uvb bulb would be in order in this case??
But anyways, then I started thinking like you, how could I just kind of block off the bottom half? I thought of just propping stuff up so the plants are higher, but if not sort of solid, won't the bugs just crawl down to the bottom half?
Like I thought about some sort of plastic shelving system, you know sometimes you can just stack several short ones? then if they were like a slotted (or with holes in it) kind, you could kind of fasten some screen or net type material over it to try and keep the crickets in the top half better. Then put a drainage pan rubbermaid thingy or whatever underneath like on the second shelf? To catch the majority of the water from misting. A lot of people do something along those lines outside their cage, underneath it, I'm just kind of incorporating that idea on the inside.
I would say just put a bin with screen over it on a shelf but then you wouldn't want to place your plant pots on it, cause then you'd have to move all those wet plants every time you had to empty the bin, and unfasten the screen from the bin to scrub it, and all that stuff. It would especially suck to have to move the plants if the chameleon happened to be perching on that one at the time.
Obviously it's not totally bug-proof, but well you could just reach down to the bottom periodically and scoop up the strays and plop them back up in topsville.
I haven't entirely thought any of this through, lol, and I'm really really handicapped in the craftiness department. But those were the types of ideas I was coming up with.
Anyways, heckle away crafty pro's! We're both looking for better (but in my case only requiring the tool-using skills of a 5 year old!) ideas for this situation.

- Melissa