Just happen to have a micrometer on my desk and my razor drawer isn't too far away while waiting for my coffee machine to warm up...
The plastic "bulb" is octagonal and a touch under 15mm across flats and a touch over 16mm across corners while the numbered dial it's attached to is 12mm diameter, the rest of the fluted handle is 12.5mm at it's widest point.
If you, or someone you know, has the equipment (bench drill, HSS bit etc.) it wouldn't be much work to open one of the ES ones to fit. Come to that, I think ES has them made specially, and might perhaps have one altered to your requirements.
^ I think the $35 shipping fee from the Land of the Free to Blighty might preclude your offer; however, Paul (@Blademonkey) is sending me one that should do nicely. I never thought a fellow Yank would ever earn the epithet of a 'Right Charlie', but there you are. Bask in the glow of your achievement.