I think offsetting the support arm would work as well, it's only a 1/4in after all.? You have a little misalignment with the bottom attachment point but it does not look like enough to bother with.
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I machined a groove into the top of the vertical column and placed an M6 bolt with the end turned down to 5mm into the casting.? That way the top can rotate but not come off?unexpectedly.
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