I used a product called Apoxie Sculpt. It's an epoxie that hardens like rocks and is completely safe for fish and aquarium use once it dries. Soon after this picture I put some 'test convicts' in there with them to see if they were remotely toxic. The bastages reproduced.
I even took some of the product in while it was freshly mixed and put it in a bucket with some of the convicts... they survived with no ill effects. Now to their credit, the convicts are still alive and thriving in a freaking brackish tank with a SG of 1.010. Who knew convicts were so hardy?!
The algae did grow on the 'roots' and it looks amazing! I have two primative looking videos up on you tube so you can check them out.