We have 2 great options for an organic mattress outer. 
You can get an organic cotton simple outer that zips on 3 sides to make your mattress. Or you can choose a fancy organic cotton quilted with wool outer that looks like a deluxe mattress. 
In any case, just fill the cover with 1, 2, 3 or 4 of our 3" cores to make it complete. Then add anything you like: Wool Moisture Pads, Toppers, Wool Mattress Pads, Organic Cotton Quilted Pad...
Zipper is tight teeth nylon with a metal head.