Can you use a kreg jig on mitered shadow box made from 1x 6?
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Can't see if anyone has answered yet, but it would be difficult to do this as a shadowbox. A frame, yes, but here it would be difficult to determine how far a screw would go into an endgrain. But maybe this can give you ideas, although it shows it from a framing standpoint.
another set of ideas for miter joints
I would suggest using splines for miter joints for strength, and it also looks nice...