I am using a 3" Southdown DSB, but just moved the little guy in, and he rally hasn't burrowed much yet.
He's got a great piece of dome-shaped Live Rock in there that has tons of holes/tunnels in it, and he basically fixes up the main door, and that's about it. He camps out in some of the "hammock"-like nooks inside the cave & has plenty of room to scoot around inside.
talk about a snow storm, though..I need a few more pieces of LR: southdown is so fine, you can usually see swirls of it on the open areaas of substrate (kinda cool, though)
On a 10 gal FOWLR that I have, I went with the same substrate, and my pistol shrimp, Randall's goby and Coris Wrasse are haveing a blast mving the stuff around....Main problem, though: too fine, so it causes "snowdrifts" and the sand is tough to remove from some of the rocks' porous surface...looks pretty natural, though.