I was recently commissioning a 2M vertical antenna that
had an in-built SO239. However, the thread on the SO239
was shorter than that of a PL259 to the extent that even when
screwed fully home, the cable was not locked (Usually by
those triangular pips on the end of the plug locating into
the v-grooves on the SO239) and could
swivel around with a correspondingly poor contact
for the braid/outer.

The solution was inspired by the new bathroom which
we installed recently, in particular, the use of (Yorkshire?)
compression fittings with their olives to join lengths
of copper pipe.

The method is to completely unscrew the PL259's nut off
the plug, and to wrap some braid off a piece of scrap coax
around the cable end of the plug. When the nut is now
tightened down, the braid is compressed and acts to lock
down the PL259 by reducing the length of screw thread that
is available to mate with the SO239