Augustine, the enclosure is really beautiful! You're off to a great start!
Your tree appears to be some type of Draecena. If it is, I believe that while the trunks will slowly grow thicker, the leaves won't grow sturdier, so they'll always sag under the weight of a chameleon. I don't believe its growth rate is very high due to its woody trunk, as woody plants tend to grow more slowly than vines, etc.
Ficus benjamina has a relatively low growth rate as well, for the same reason. However, you could buy one now and begin to grow it up if you liked.
Scheffleras (Schefflera arboricola being the one most used in chameleon enclosures) grow somewhat faster, yet the stems that hold the leaves tend to be rather thin. However, many people use Schefflera because it provides lots of hiding places for a cham, and as it grows, the lower stems thicken and become sturdier.
As Carol5208 suggested, your best bet is probably to incorporate some artificial vine going up and across the enclosure in several places to help fill in open areas as well as to provide a basking area at the appropriate level. As your Epipremnum aureum (the golden pothos) grows, you could wrap it around the artificial vines to give your cham more hiding places.
Good luck with "finishing" (I use that word a bit tongue-in-cheek, because from what I'm seeing, there's no such thing as a completely "finished" enclosure for a cham; they are always in process!) up your enclosure!
Sandy
