I believe Ton Kooiman's Forza thumb rest/hook can be purchased with a plate that can be soldered to your horn after the current one is removed. They are not cheap, however, $150.00+. After removal of the original and installation of the plate, the Forza is screwed to the horn.
you can indeed modify a metal thumbrest but it will deface your horn and change its looks permanently (especially if, as I suspect, your horn is silver or nickel finish). So what is exactly that you are seeking to achieve? If it is too hard you can accessorise with a cushion.
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