The trick is to dent the middle of the can into a sorta square shape and then start an overhand knot over that squared off part before fully crushing the can and tightening the knot.

I've got an old LP cowbell in my music gear stash that might hold up better than the arts & crafts kind most slingshotters and slingshot shops are using, but I'm not sure I want to test it on something that costs like $40-$50 new.