There was a fellow from the UK that constructed and marketed wall thickness gauges. IIRC, his name was Bob Brown and he was a friend of the late Bill Wise. I met Bob when visited the Southern SxS event years ago. Don't know if he's still in the business of selling shotgun accessories; maybe a search will turn up something.

BTW, the link to the Galazan gauge posted above gives me a 404 message using three different browsers.