Use a screw anchor, to chain, to a rope that has a loop through an Eva Fender. The rope/loop/eye at the top of the fender.
Mooring balls are a bit hard (they are all hard shell) and can leave a mark. You could use galvanized threaded rod through the Eva, but something heavy like 3/4 line and an eye at both ends with no slack on the fender would make the fender into a friendly buoy. It would stand up and be easy to identify.
You could get away with a 6-8" Screw Anchor.
Eva Fenders - something like the 31 x 10