It is best not to use any substrate on the bottom of the cage.
Chameleons hardly ever go to the ground, so it's far more practical to leave a bare surface that makes it easier to clean the cage.
However, if you don't have any form of drainage for all the dripping/misting you'll be doing, some people would recommend using paper towels to absorb all the moisture. If you're going to use the paper towels, make sure you replace them regularly to avoid bacterial growth and/or smells developing in the cage.
Also, remember that any substrate is a potential hiding place for any feeder insects that you might be free-ranging to your cham.
Coco fibre makes an acceptable substrate in my opinion. It is finely shredded enough to avoid impaction. As long as it does not get water logged, it can do a great job of retaining humidity. Spot cleaning is required at a minimum of once every second day.
I personally don't use anything on the bottom of my cage because it has a white plastic bottom, my cages are surrounded with lush tropical landscape both inside and out so substrate or paper towels get wet and dirty too quickly for me to keep up with, instead, I cut a small hole in the corner of the cage very low so my chameleon can't escape, and every few days or so I'll blast all the poooooo and dead crix out the corner with the hose, and about once ever 2-3 weeks, I take my came out and scrub his cage and vertical playpen down with mild soap and diluted bleach.
I'd also suggest no substrate, so you can monitor your chameleons intake of crickets. Even if you cup feed, I still recommend no substrate. It makes life a whole lot easier on the chameleon keeper.
I used to use substrate, but it was very messy.
I now use j cloths, like papertowels but they absorb more water without falling to pieces, and unlike towels cham wont catch its claws on them, and u can wash them