We had a table mount DeVere 504 with the table removed in an almost identical situation to what you described. This was late eighties so the unit was an early to mid eighties build.
What we did was to weld a socket to a universal socket (UJ) joint from a normal workshop. To the UJ we drilled and bolted a steel shaft with a T piece ending.
We also had the socket and nut head scored so that they only became married to each other with a fair amount of pushing. This meant the set-up stayed put whilst we were focusing and that sort of stuff, but we could easily pull the arms off if required.
Luckily after welding the socket and UJ together, we discovered that the steel arm was able to hang down in a near vertical manner. I think you will need about 6mm rod as a minimum thickness as mild steel twists easily. We found this out with our first one when we cobbled it together rather quickly.
This enabled us to sort of emulate the UJ jointed arms on the huge Dursts we had.
Mick.