After some trial and error, went with a half-inch square glued to the bottom of a standard Terrain ground square. I had experimented with having a shaft glued to the bottom that went down the gap in the corner piece to serve as an anchor, but it wasn't very strong and I didn't find it was worth it. One thing I might change is to use the same spike strip that tops the wall, for both consistency of sizes, and time-saving (since pieces may already be cut). Suggestions on improvements are welcomed!
