Why not make something a bit more appropriate for a chameleon, perhaps the trunk of a tree?
Being that this background will serve absolutely no purpose/function for the lizard itself(I cannot picture a chameleon-regardless of which species utilizing/climbing a rock facade), and is purely for aesthetics, why not make it something which matches the type of situation that these animals occur in? A tree trunk can be formed in the same manner as a rock wall can, and you can even utilize the same materials as well.
Another option for your chameleon, which would benefit the lizard as opposed to just yourself, would be to render the walls of the enclosure usable/climbable for the chameleon. Instead of making a rock wall which could not be used by a chameleon with zygodactylic hands, why not affix material/s to the wall which will increase the total amount of surface area available for use by the chameleon? The walls of an enclosure are a goldmine of prime real estate for arboreal/climbing lizards, but are very infrequently used to the captive's advantage. Your chameleon will greatly benefit from the increased surface area which it could use in its daily movements, as it would no longer be restricted to the up or down range of movement that most people restrict their lizards to by only providing a few branches within a large, spacious enclosure.
I do not see the practicality in making a rock wall/background for a chameleon. There are many other alternatives which you can use to "spruce" up your enclosure, allthewhile making it better and more functional for your captive. It is all too often that keepers jeopardize the functionality of an enclosure, by trying to make it aesthetically pleasing for themselves (reptiles couldn't give a damn about aesthetics, as long as the tools and resources are there for them to use towards their physiological needs). There are ways of acheiving both, however you must take the ecology and physiology of these animals into account when designing a functional enclosure.
Just my thoughts on the matter.
Good luck,
Bob
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