"Next Shubb problem is the overcentre mechanism. Some 12 strings have necks like baseball bats to reduce neck problems. And then when you try to clamp the Shubb it does not quite get over-centre, you play two chords and off it flies to hit a person in the front row in the eye. Why don't they make ones for fat necks?"

They do - the S3V

I have one which I use on my Lowden and Martin OM-28 Marquis (1-3/4" Mod-V neck profile) - works fine on those, but I do not have a 12-er, so I can't comment on its suitability for one of those.