ok here is my idea that i think will be solid (hope)
we use this
Like you said but then we make the adapter that goes in the gun and drill it for the 1/8npt fitting. Then all we have to do is screw it in with some Teflon tape.
seems simpler to me but do you have the tools to mill that piece?
I might have access to a lathe.
But nothing to thread the 1/8 npt fitting.