Hrmm...
In line with Iodo's reasoning, I was doing some research on the great "outdoor gear revolution" of the late 19th-early 20th century and came across the manner of thought that became the M1928 haversack. It's basically Iodo's picnic roll, with a bunch of extra buckles and straps to hold everything in place, and some shoulder straps sewn on one face. Here's someone selling a reproduction with good pictures:
https://www.atthefront.com/product-p/usgbpm28c.htm
From what I understand it wasn't well thought of at the time compared to a proper bag-knapsack, but certainly did the job well enough.
I think if I were to render that thing of Boromir's, my first attempt would be something along those lines: a great big primary panel, with side flaps sewn to it on at least 2 and probably all four sides. Flaps hold things in, then the whole thing would roll up blanket roll style for security. But I think a toolroll construction meets the available evidence just as well.
I don't think ether would be as handy a solution as a proper bag, but both fit the visual evidence better.
FWIW, I've had some luck hearing back from WETA just asking for info. Have you tried contacting them to see if anyone remembers how that item was made?
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edit - gah. no, looking at the side material again on a bigger monitor, it does look the same leather as the roll. And it wouldn't poof like that if it was a roll, as opposed to a portmanteau.
Hrmph. I'm confused.
![Confused :?](./images/smilies/icon_confused.gif)
Persona: Aerlinneth, Dúnedain of Amon Lendel c. TA 3010.