For a while in the 1990s I tried to make a business making 'Hardangar' violins and electric violins.
Hardangar violins are a Norwegian specialty, coming from a town of that name, and have four extra strings running from the tailpiece, through the middle of the bridge (where they sit on their own bridge), through the middle of the neck (under the finger-board) to a nut set underneath the main nut, and beyond to their own set of pegs in the peg-box.
The electric violins shown had a layer of rubber set into the bridge to give a 'semi-acoustic' feel to the sound.
The bath-tub cello was made as a prop for a cabaret band, and plays surprisingly well, but sounds a bit more like a bass.
