Abstract: A cap is provided for mounting a light fixture to a post having a longitudinal shaft of rectangular section extending therethrough. The cap includes a closure for the shaft bore having a drive cone secured to the closure and extending into the shaft interior. Fingers are mounted to the closure for transverse movement within the shaft. Each of the fingers has an inner edge abutting the cone and an outer edge adapted to engage the interior of the post when the finger is shifted outwardly. A drive screw extends through the closure and serves to drive the cone upwardly and hence the fingers outwardly.