Found a better option
Let's be honest here: 5/16" is one horse-sized urethra and probably gonna stretch if not blow out most guys. Anyhow, we instead bought a set of Hegar 8 Inch Urethral Sounds from here, which come in sizes from 3mm on up in baby steps (search for Hegar to find them). I worked through the first six levels, about 1/4", or two full steps below the 5/16" of this unit. Best of all we just taped the electrosex lead to exposed end of the Hegar sound (and attached the other end to a penis band or tens pad on the scrotum) and voila! Custom-fit electrosex urethral sound for less than the cost of this unit.