I take the old arm board to a plastic fabricator in town and have a new one cut, in plex, with no center hole but the screw holes on the perimeter like the original board, usually in half inch scrap stock.
I then take the board home, bolt it to the table and measure to the center of the arm hole, and then take it back to the fabricator and have them drill the arm hole around the marked center. Last one I did cost $28. Good luck should be a nice rig when you are done.