By 1969 the end caps were a plastic (or fiberglass) formed shaped segments - that's why there are no support ribs. They spanned the width and were attached at the rib and window frames. The exterior caps are also constructed out of shaped (stretch formed) sheets - they aren't flat, so using flat aluminum to make the inside curves is tough. The older (pre-1958) 13 panel end caps were flat segments that are riveted together...those are most likely the interior segments that you've seen.
Some folks have overlayed aluminum sheets (the more segments the better) to get the "look" of the older trailers but I don't think there are segment patterns available. Besides, every trailer is a little different anyways...can it be done, yes...but it's not easy figuring it out - kinda trial & error. I would say making templates out of thin cardboard would be the way to go ~