I have a 12-string made by Arturo Valdez, "repairman to the stars," and luthier.
He had a shop on Sunset Blvd, did work for everyone. He made less than a dozen 12-strings (including one for Roger McGuinn), with the same body dimensions as the Guild F612.
(He was also John Denver's repair-person, and re-topped Denver's F612. Curious that when he designed his own 12-strings, it was with the same dimensions as the F612, though it resembles the F612 not at all!)
Anyway, the case Valdez used appears to be a hardshell acoustic bass case. It fits the body of the guitar perfectly, but the neck area is much longer than any 12-string neck, even with the typically elongated 12-string headstock.
The guitar and case are from the '70s; not sure if the exact case is still being made, but you might check out acoustic bass cases as your first option.
Good luck!
Glenn