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.