I'm thinking about quite different approach. Instead of making a removable adapter it's better to make a permament insert. What I mean is an insert that will screw into the sparkplug hole from the inside and set there forever (not sure if loctite would work). The depth of the insert would be the length of the glow plug treaded part. This way the glow plug would go straight into the cylinder and there will be enough spaca between the glow plug and the old spark plug's thread for you igniter or wrench. Something like ere below.