We used to keep old spark plugs around to break up the insulator and use the metal body to make adapters for leak-down testing, back when different diameter plugs (other than the 2 common diameters of the time) first started showing up. Sounds like that's still a good idea - I'll have to check, now that you brought it up; I'm not sure that my leakdown test rig has a 10mm adaptor either.