Hi D-man
Is the aim of the exercise to just blank the bore of the fitting ?
If so depends on the pressure it is going to see, but you could make a expanding bung. it's just two circular disc's bolt welded in centre of one and a clearance hole in the other with a doughnut shaped soft sponge rubber ring between the two, tightening the two plates expands the rubber and locks the assembly in the bore. Plump Centre stock this sort of thing.