I followed Dougal's modification to EPA's idea which doesn't require the 57-50 mm ID reducer. Unfortunately, I overlooked his identification of a Mk 2 MX5 filler and bought a Mk 1 filler whose pipe is the wrong diameter for this application. If you're in the US, the Mk 2 is the NB generation of Miata manufactued from 1998-2005. Inserting the 50-45 mm ID reducer hose into the fuel tank pipe did not seem to be as tight as Dougal describes but seems secure enough to do the job.
My filler pipe takes a 90 degree turn at the bottom unlike those of the pipes pictured by other posters. The hardest part of the whole process was getting the nitrile hose fitted between the 90 degree pipe and the pipe into the fuel tank. There is very little room between the two pipes in which to maneuver the hose into place. I've attached a couple pictures to illustrate the pipes on my '71 Plus 2 S.
I'll have the car back on the road in the next couple weeks and will report back then.
Lee