Yeah, I was thinking of putting a bromeliad or two into the background, but was also considering a larger one on the substrate. (Using and 18x18x24) Although I suppose I could make a small mound out of the same stuff as the backgound, to place on top of the substrate.

I was thinking of something like this for the drainage etc: http://www.majhost.com/gallery/Sykur...ium_design.png (I only have paint so it is rather crude) Obviously with substrate and plants on top.

Also, do they plants need watering? Or will the humidity be enough for them?