Interesting idea, screwing a round-headed screw into the bottom of the stick and fastening a recessed gizmo (liker a bottlecap) to the washtub/soundbox in which to rest the stick.

The way I handled it, back when I had a washtub bass years ago, was to cut a V-shaped notch into the bottom of the stick and rest it on the lip of the inverted tub. The kind of real galvanized washtub you could buy at a hardware store (or salvage), at least back then, always had a ridge around the bottom which nicely accommodated a notched rake-handle or whatever you were using as the "neck" of your homemade bass.