We recently replaced all the interior panels in our 1953
21' flying cloud.
We created a spread sheet with the measurements for each panel (excluding 13-panel end caps).
The panel labeling is corresponding to our reference photos which I will update this post with soon.
Hopefully it will help someone trying to replace interior panels on their trailer.
Here's a link to the public google doc.
We purchased new end cap pieces from Colin Hyde Trailer Restorations
he cuts and pre drills holes into the end cap pieces and we sewed/riveted them together to create a new end cap.
We got all our new aluminum from Air Parts Inc
they are super helpful on the phone and will make the ordering process very easy.