As luck would have it, I recently spent a few days working on my lathe.
I have been utilising some of my surplus materials kicking about.
Just happen to have 4 sets of harness bosses up for grabs
They have been made from 19mm bright round bar & have been bored n threadded with the required 7/16" UNF thread, 50mm in length.
£7.50 per set inclusive of postage.
Drop me a PM if you are interested.
Failing that, try Saturn or Talonmotorsport, I am sure that either of them will be able to help out