Put a bolt inside. You can add nuts to bolt head which will spread the load more evenly. Tight hole will keep bolt inside. You can push it inside or pull it by a nut from outside.

Low $fn will keep angular cylinder which does not slip so much.

Notes:

Print it upside down on the flat area. Add a brim to avoid warping. Bolt bed is modified to be printed perfectly but you have to set same layer height here and in the slicer.