🚀 go-pugleaf

      It would be nice to be able to keep the cover files with the interior
      page files together in one file.




Nice for you, maybe, but almost certainly a pain in the neck for the printer. The entire cover, in this case, could have been done as one large page, divided into appropriate panels, and fold guides indicated, and simply left as a separate file.

There's nothing unique about spiral-binding and fold-outs. ALL foldouts need to have a narrower page panel to allow them to open, and they always need some sort of special handling in the folding and trimming, so what you did was quite normal. ANY TIME you get into special binding situations like this you should ask the printer how HE wants to handle it, preferably before you invest a lot of time in a layout that won't work well.

Peter