This is the one I got for Imperial Assault. I think the 7771 would actually be better for Legion because the internal trays are just trooper-sized. The 3650 and 3455 boxes are taller, which fits some of the weirder-sized Imperial Assault models but results in having one fewer tray.

I didn't see in your pics, does the base and stand for the airspeeder fit...in the top? The lower right compartment?