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Great idea! I like the plywood construction seen around the various tools in your shop. Did you consider adding any feature to ensure the caliper is mounted squarely? Perpendicularity could potentially affect the accuracy/repeatability of the measurement. If I were to make one I'd try adding two pins in between the magnet mounting holes and offset them from the magnet holes centerline by half the caliper width. Then simply slide the caliper against them to square it up.
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