The ultimate Macguffin would, of course, be something where neither the characters nor the audience ever finds out what it is. I don't think it's going to get that far, I do intend to open up the final box (it involves several boxes, each of them magically sealed for prevent its being opened without certain conditions) -- but the reason why a villain wants it is that it was confiscated from a relative, and the reason the relative put a box into a magical box, was that he could not open the first one, and by locking it away he could spite the owner of the first one.
So, a Macguffin. But only a penultimate one. I may even decide that its contents are plot-significant yet.