A slingshot with a knuckle duster-esque grip. All you need for assembly is super glue/some sort of strong epoxy and rubber band(s). You may need extra glue or a file/sandpaper depending on your printer's tolerances. Just add glue, and glue the handle in the slot on the arms of the sling, then thread bands into the shot cup, and you're done!
This thing was made with Tinkercad. Edit it online https://www.tinkercad.com/things/imR1c0r6FLm
35%+ depending on desired strength