Feeling creative one day I dowloaded a model of Fi, opened it in Blender, removed the insides and scaled it down to the head. I also cut open the mouth as I've learned from personal experience that computer paper doesn't breathe.
It's 291 mm. tall, slightly shorter than the Jenova mask, so it should probably be able to fit an adult head, though I haven't had the time to test-build it yet to see if it actually fits.
Also a few days after making and unfolding this I found that there's going to be a Fi mask item in Animal Crossing: New Leaf. Ain't that a coincidence.
Warning: lots of small parts.[link]
Update 06/01/13: Slightly scaled model up, fixed overlapping parts, changed from A4 to letter.