Theo,
I don't think that my setup got used, but I had over 6" of the barrell wall sticking up above the pea gravel. I thought that they would like it, because they would only have the up direction to defend, but I think they might have felt boxed in. If you do it, you should keep the edges 1-3" hihg, like Bill suggests. After my pond enlargement, I now have some flat areas, where I don't have to worry about the gravel sliding off the tarp. Also, it will provide environment for more natural nest colony.
