I've discovered I can easily load and unload my 18x24cm plate holders in a changing bag, while 5x7" is about the practical limit with "modern" holders. The plate holders are book-type, and use sheet film insert sheaths. A dab of spray-on "yellow sticker glue" in the middle of each sheath holds the film secure and flat without making loading/unloading any more difficult.

Maybe that could be adapted to ULF too?