They like being on rocks but are usually in crevasses and such. 
To make mine movable without messing around with their foot all the time, I got a piece of flat rock that has a dip/pocket in the center and put that on the sand. Once they attach you can move them around. 
 
You can also take a dremel to some rock and create a pocket. 
		
	
		
		
		
		
		
	
		
		
	
	
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