Hello.
I used to have many veiled chameleons in the late 90’s, and for the first couple of years I had many cages that were too narrow (such as 2 feet by 2 feet by 4 feet tall). After years of building many different shapes of cham cages, and after meeting other keepers, (many from the CIN or Chameleon Information Network), I started to see that there was a better cage configuration. I have found that the best seem to be not only high, but wide. A typical veiled chameleon, just like other chameleons, likes to be high up, but also needs to move around horizontally.
So, a 24x24x36 might be suited for the smaller species, but is not best suited for an active veiled-sized cham for many reasons. It is a common misconception that height is the only important parameter. Chameleons seem more comfortable at higher levels, typically at least at human eye-level, but 2 feet by 2 feet does not provide ample space for a veiled chameleon to move around horizontally. In a 2x2 cage at the top, a cham can easily be forced to move downward to thermoregulate, instead of being able to move horizontally away from the heat source.
In the wild, chameleons do not move to the top of bushes and trees and stay there, they move around horizontally once they reach a comfortable level.
So, a great cage would be 4 feet wide, at least 2 feet deep and three feet high, with legs that put the top of the cage at about 6 feet or so from ground level. A cage like this sounds huge, but it really is not and can be built yourself and can be a fun project.
Believe me, I saw a big difference once I switched to this type of design.
Another consideration is cricket-control. Typical mesh used for cham cages will allow your feeder crickets to escape into your home. On the other hand using screen will stop that but can break your cham’s toenails if they decide to walk up it. A good solution to these issues is to use mesh on the inside and screen on the outside. The downside is decreased view of your animal. However, a solution to that is to add a glass door (to the front only), so that you will still have appropriate open-air flow around the sides, back, and top.
Good luck!