So here is what I came up with. I had some problems with the large shelf piece as it was too heavy. I ended up having to use 5/8" PVC inserted into a piece of acrylic with a 5/8" hole drilled in it. I then glued the PVC to the acrylic sheet. Also, I drilled 5/8" holes through a heavy rock to use as the base and through all the small rocks and threaded them on until they were almost all the way to the top. I drilled a shallow 5/8" hole in the shelf and placed it on. I then used some epoxy to level the bottom rock so it would not wobble and used epoxy to steady the shelf piece.
For the small tower on the right I drilled a 3/8" hole in the bottom of the rock and placed it on a 3/8" acrylic rod connected to a piece of acrylic. I then drilled a 3/8" hole in the top of the rock and the bottom of the larger rock. I then just stuck them together using a 3/8" rod. I still need to use some epoxy to stabilize the bottom rock.
Chad