I use the tubing recommended in a thread on the Backpacking Light Forum. It isn’t necessary, and adds a couple grams, but it is a nice luxury that allows the mug or pot to be plucked from the stove without additional precaution.
Make sure to use silicon tubing, not vinyl. Application takes some finger strength. Spraying a little soapy water into the tubing first makes it slide a bit easier. The stuff I use has an outer diameter of about 5.8mm and an inner diameter of about 4.2mm. It has been the appropriate size for the handles on mugs from Snow Peak and Boundless Voyage.