try an aluminum shim - out of soft aluminum - such as a soda can (side) or
beer can (thinner) - cut into a strip that goes once around the shaft, press
on the wiper arm and then take it back off.  clean the shim and the inside
of the arm, put a little good quality epoxy on the walls of the arm to hold
the shim in place and press it back on.  If you grease the knurled spindle,
the epoxy won't stick to it.

I think I have spare wiper arms, but not a rear one.  (there is a list of
parts from a rather beautiful car that suddenly became a parts car on my web
page, wbnoble.com)