Abstract: A fence post cap is fabricated of thin-walled material with a cap portion above a plurality of depending walls. A first wall has an inwardly and upwardly extending thin-walled first protrusion with a first upper edge providing a mouth between the first wall adjacent the first upper edge and the first upper edge. Likewise, a second side wall, opposite the first side wall, has an inwardly and upwardly extending thin-walled second protrusion with a second upper edge providing a mouth between the second side wall adjacent the second upper edge and the second upper edge. The protrusions can be positioned so that they are opposite one another such that a shortest distance between the first wall adjacent the first protrusion and the second wall adjacent the second protrusion is greater than a shortest distance between the first wall inwardly extending portion and the second wall inwardly extending portion.