Can you do the penny in the clutch/ring gear trick? I may have a TDC tool from my 2 stroke Kart racing daze, I'd have to scour the garage to find it. It was, as you suggest, a spark plug threaded sleeve with a long nylon center post, but intended for a 2 stroke head, where the plug is centered on the top of the head. I doubt it would contact the piston in your XT500.
FR