I have three right now. I've had them for a while. I use kind of a trick to keep him in place. I take a very large snail shell like a Mexican turbo or top crown Astria plant the shell upside down in the sand with just the hole showing. Plant The Rock nem inside the hole and I have not had one move yet. They appear to disappear at night but pop out of their shells in the morning.
Instead of snail shells I've been using different size PVC end caps. The RFA plants it's foot in the bottom of the cap and I press the cap down into the sand. After things seem stable, I can press the cap down under the sand and so not one RFA has moved. This also makes it easy to move them or take them out of the tank for whatever reason. If you have them breeding and want to sell them, this sure makes that easy!