Hi, my instinct is to sell them to save on storage haha
That size is def a challenge tho. I've seen various large-format storage / transport artwork portfolio "folders" (not boxes), some kraft cardboard/paper-based and some poly, scattered around the web but I don't think I've seen anything that large. And ofc archival properties vary widely if that's a concern. Also the much more expensive fabric-based portfolio "bags" but again that size...
When I down-sized my home my steel flatfile storage cabinet was on the chopping block. After looking at all the storage options (folders/hangers/boxes/tubes/etc) and where I would store *those*, I decide the flatfile cabinet really was the most space-efficient thing (because it doubles as a work surface) so I made it fit. It comfortably holds 36x48 inch sizes with room to spare. But a cabinet that could do 64"+ would be truly massive. And ofc it's not portable lol, all-up weight 400 lbs *empty*.
I feel like if none of the large artwork folders or slim boxes are suitable, rolling might be your only option. Anything from regular paper-based tubes to the large-diameter poly transport tubes (some of which are expandable/telescoping). The latter were a competing option to my flatfile cabinet (but a rolled stack of paper prints is so annoyinggg...).