It's not cheap, but you get what you pay for. This high-quality British conductive rubber has an external diameter of 6.3mm and accepts 4mm plugs.

It has more flexibility than standard 6mm conductive rubber and more skin contact area when worn for a smoother ride.

Its specifications are:

• 6.3mm External Diameter (+/- 0.15mm)
• 3.2mm Internal Diameter (+/- 0.15mm)

Sold in one-meter lengths, you can then cut it down to whatever size you need.