Morgan,
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I have made wooden blocks similar to what John showed, milling a long piece to have spare parts as required, as Jonathan mentioned.
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The width of the top of the groove on the Hammer is 11 mm, so the block is slightly narrower than what John shows. Threaded inserts work, but you need to be accurate otherwise parts of the thread will punch through the sidewalls. Should that happen, get rid of the problem using a file.
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J.
