Put your electroconductive rubber cord and tubing through these in any configuration, the circuit will complete only when in contact with the skin so you completely control the sensations and current. Set of 2!
Now in 6 varieties, choose which shape will be the most effective for you: (Flat means the hole isn't perfectly round inside)
3mm Round: can be used with 2mm cord
4mm Round: can be used with 4mm cord or tubing
5mm Flat: can be used with 4mm cord or tubing
6mm Round: can be used with 6mm tubing
7mm Flat: can be used with 4mm cord or tubing (hole is 7mm wide but only 4mm high)
9mm Round: can be used with 6mm or 8mm cord or tubing (8 is very tight but doable)