ha I know what you mean. Its always hard to tell in the pictures anyway, so I couldn't tell if they were wide or not. Looks like you have most of em just right though. You could get some masonary screws, and run them in the rocks, about half way on the vertical parts, and then you would have something for the markers to "hang on for dear life" with.
I really like the orange tennis balls a lot, they kinda stand out better to me. I might pick up some to try, all mine are yellow.