I am not familiar with Michell TT s, but what is so special about the arm board that prevents you from finding a machinist who could make one to your specs? It’s probably just a matter of drilling a hole in the right location on a blank arm board.
Also, check spindle to pivot distance for the arm you want to use vs P2S for the available arm boards. You might luck out and find that one of them is actually useable with your chosen tonearm.