You're six inches shy of what I think the ideal is...that's not so much. Make sure the enclosure is really full of live plant(s). I have one giant pothos hanging in the middle of the cage and the vines are pinned around the enclosure. Lots of leaves to hide behind seem to be a good thing (plus, I get so much "where the heck is that guy?" time....I mean, look at him! He's the size, shape and color of a leaf).
One thing that might make it more likely to succeed is draping 2 or 3 sides with dark fabric so the chameleon can't see what's "outside".