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I am lining the inside of a hope chest. The external panels are screwed from ply on the inside and the unsightly mess will be covered with tongue and groove boards. I have taped the rods to the ply to take a picture. Can you see the mark across the two rods?

I do not know how to measure the internal dimensions accurately except by using pinch rods. These are just thin laths ( from the T & G) pointed to get inside the corners.
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Then the rods are transferred to the mitre saw.
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Internal mitres are usually scribed which is more difficult and this will be quite satisfactory.