Sounds reasonable, you could get the tub threaded for the 42mm and just tighten it in place. If gluing it in place, a slow cure epoxy would work well, just degrease everything with a solvent first. Or, just use the adapter ring as a nut to hold the lens in place in a 39mm hole.
Edit: Just for your info, two full threads of 39x1mm threads will work without binding on the lens barrel too much, much more than that and the lens will jam in the threads before it is seated.