A couple weeks back I asked the IDENTICAL question. I was at work, reading lando's response (same as this one). I set mine on a rock just like the lfs
When I got home, bada-bing! There he lay; right on his face! He must have tried to move a bit, or maybe just the swelling may have caused a shift - whatever, it caused him to fall off of the rock. I managed to save him in time but now he resides on the substrate and loves it.
Keep in mind, they swell 2x/3x their contracted size. Mine is about 5" diameter at night. During the day, he's over twice that.