It is really not that hard to do at all. The more open your rock is, the easier it is because the less you need to drill. I used (mostly) pukani rock from BRS, thorlite cement from marco rocks, and 3/8" acrylic rod from US Plastics.
I built mine up a rock at a time based on a rough vision of what I wanted. I'd take a rock that looked like roughly the right shape, see how it fit, cut an acrylic pin the right size and fit it into holes in the rock. The pukani rock has enough holes so a lot of the time I didn't have to drill it and you can shape it by just breaking pieces off with pliers. I put the thorlite in the holes and around the pin and joint, letting it seep into the openings in the rocks for a strong bond. I zip tied or braced the rock while it dried. It took a while because I had to add a rock, wait to dry, add a rock, wait to dry, etc.
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