Not cheap, but you get what you pay for. This is high-quality British conductive rubber that has an external diameter of 8mm and accepts 4mm banana-style plugs.

You can insert a short length of 4mm external diameter CR to be able to use TENS style 2mm plugs with this conductive rubber.

Sold in 50cm/20" lengths you can then cut it down to whatever size you need. If you order more than 50cm you will receive a single length of conductive rubber to the length of your order. You can make two 25cm/10" cock loops from each 50cm length.

The advantage of using 8mm CR over smaller diameters like 6mm or 4mm is more surface contact area which leads to a smoother sensation when used with conductive lube/gel.