The Horizon/Zenoah adapter places the glow plug in approximately the same position as the spark plug as you want it to be. (And as it should be.)

A standard glow plug wrench will JUST fit, but it's tight and it doesn't fully engage the hex. I just use a 1/4" drive socket, ground down on the outside to fit the recess.

Another way to do it is to make your own adapter out of an old spark plug. It's a bit tricky as the shell gets pretty thin as you mention, but I've made a number of them. After gutting the plug, I braze the end shut, drill out the internal recess for socket access, mill the braze flat in the recess for the glow plug, and then drill and tap it for the glow plug. All in all a lot of work for a part that's available for 10 bucks... You will also need a special tap, as glow plugs are a somewhat rare 1/4" x 32 thread...

AV8TOR