Pinch rods supplement: internal mitres-1097f308-f83a-4fd0-b061-000806f83113.jpeg
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.
Pinch rods supplement: internal mitres-994402ba-8af0-4e5b-8142-73188cce50fe.jpeg

Then the rods are transferred to the mitre saw.
Pinch rods supplement: internal mitres-7cf84067-6d0b-4c93-80d9-41bf9721ee1b.jpeg

Internal mitres are usually scribed which is more difficult and this will be quite satisfactory.