I used pond foam with what I'd consider good success. Granted you won't get the gravity defying displays you see with the marco rock, but it holds the rocks together after you've stacked them up. The only downside for me was it expands more than I thought it would, and I ended up having to trim it in a few places. If you try it, go slow with it, try it in a few spots and get a feel for how much it expands before doing too much at once. It will also pull apart fairly easily when and if you need it to.