
I thought I understood the concept reasonably well but I was not confident enough to hack into my nice doors straight up. So I spent a few hours making a 1/3 scale mockup. This let me confirm my understanding and try out an idea to avoid needing drain holes. I have a routed a 45deg chamfer on the bottom edge of the opening and continued that angle on the backing piece.
It also allows me to confirm that the frame can be a different height on either side to accommodate my D shape door and opening.







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