I have discovered a red rubber bung 40mm long and of 43mm diameter tapering down to 34mm. These are readily available on Ebay. I haven't been on a run yet with it but I don't expect any problems and it must be a big improvement on the screwed-up plastic bag I have just rode 3000 miles with after losing my original cap somewhere in Belgium or France. I'll let you know. I intend to attach a line to it so it doesn't get lost and it has the GREAT advantage of fitting and removing with one hand. I have no worries about fuel-theft nor sand (etc ) in the tank.