Dead Blow Hammer. Fill with your own media, be it sand, ball bearings or lead shot. When you hit your target, the media will shift inside and add extra force to the blow. The threaded cap is tight, adding a little grease or TFE Paste will help it screw on. You'll also need to glue the handle into the head.
Rotate the parts so they have the flat face towards your print bed, and print with no supports. The inside is supposed to be hollow.
Print with a minimum of 20% infill, though 50% is probably best and 100% is not needed.
Sorry, no matter how I rotated the parts in Fusion 360, they always came out in the same orientation when converted to STL. If anybody can help me with that, thanks in advance