A 3" deli cup works good for me, I place it low in the cage and have a branch hanging partially into it. I also taped a piece of paper covering the back half of the cup so my chameleon doesn't try to catch food thru the plastic.
I just read in my care book about this....it says the cup should be 6" deep and secured to a branch by a curtain hook or I'm going to use an 'S' hook. It should also be opaque so that the chameleon won't see through the sides and try to feed through the sides. I hope this helps you.