I used picture hanging cable, the brass looking stuff, as it was thin and corrosion resistant.
|The cable and fixings. The 'modified' connection is on the left see below|
To achieve this, a hole was drilled in the bottom plastic part of the cap, taking care not to affect the locking mechanism. The cable was fed through the hole and secured to itself using a modified electrical connector. The 'modification' is a pompous way of saying cut off the connection part and discard it!
|The cable attached to the black plastic of the filler cap|
|The filler neck showing the hole and channel|
|The finished tether|