A couple of tips I have come up with after doing over a dozen cabs...first I have tried to make similar jigs to drill corners holes for leg bolts after assembly and found out its easier to do before assembly with a half round router bit. Mark and route each side seperatly before assembly for a perfect hole...
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You should also use a corner joint for strength, a locking miter joint is the strongest option but takes some practice and requires a large router table...
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