Easy--I only allow mellow and/or sedentary chameleons to stay in them.
My male veild--neither mellow or sedentary, has a big cage on the wall, on the other side of the room. I left his old cage door open one time, and he managed to assault both of my female deremensis. Guess they looked enough like female calyptratus to him.
My male deremensis has NEVER tried to escape, even when he was in the mating season "roamy" mood.
The female deremensis will occationaly get out, if I leave a plant or brnch too close to the front barrier.
Female veilds are nearly impossible to contain, and I ahve had all of them escape every day for a week. they were getting out of their cage, the SECOND fromt he window, and going to the one by the window, where the pair of sub adult deremensis were. they were fine when I got to them, although the deremensis were more on edge than usual. So I swapped them out.
It's been over a week, and they haven't gotten out since. Funny thing, since the window sill is right there, they can easily escape any time--but they don't. Seems they just wanted to be by the window. Worked out nice, since the deremensis like the shade better anyway.
Any chameleon COULD escape, if it had the motivation to. The reason is the way my barriers are constructed. They're 1x2's. I have seen them climb 1x2's with no problem. A male panther or veild will WANT to get out, and they will. What you'd have to do is use a slick, or VERY smooth material to cover the barrier supports, and they'd been escape proof.
Still, I wouldn't keep anything "dangerous" in a open cage like this--the male deremensis is as far as I'd go. A male veild, panther, or ousteleti could really hurt the others if he got out. I keep my nasty ones locked up, and I let the nice friend;y females have more leeway--they seem to respond VERY well to the lack of walls, or, more accuratly, whatt hey percieve as a lack of walls.
