mantis protectors do work, might have to put washer underneath if your metal is preventing the ball from going in the hole.
Just a few tweaks might be needed. I have em on my MB machine, but cliffy protectors work well also. Have a ton of them on my machines. I do think Mantis ones look cleaner and nicer for higher end restorations however.
HSA does great retouching work, they did my MB playfield and I could not be happier!