Eustace's "true" form at this point is a shockingly ambulatory box with interlocking teeth, meant to snap shut on anyone who isn't supposed to be rooting around in it. The storage space inside is essentially infinite, but he only really offers to carry things like this if no one can spare anymore hands. Even if the items aren't visible to those reaching inside, he can "guide" it into their hands easily enough.
He can theoretically hide people in there too, but he refuses on principle. He's not sure what would happen, what they would see, and he's frankly not in any mood to find out because he's not sure if he could get them out safely.