Try to source broken concrete - it's generally free as those folks digging up their sidewalks or driveways have to pay to dump it. Slabs work great as crays can tunnel under the shelf and remain protected. You don't need piles of it, either, just a couple slabs placed in 2-3' depth every 15-20' would be more than enough in my experience. Don't underestimate your vegetation structure/cover also. If you have abundant rooted vegetation it will provide a lot of predation protection from SMB and other predators. Of course use all the bricks you have on hand, just letting your know concrete works great [learned from Cody].

Many men go fishing all of their lives without knowing that it is not fish they are after. ~ Henry David Thoreau

[Linked Image from i1261.photobucket.com]