I sometimes carry the neck inside the horn's bell, as is the situation required by the Soundwear case reviewed in another blog. The Soundwear case (now my favorite) comes with a polar fleece type of neck bag, which is just barely adequate. The separate Soundwear mouthpiece container has a metal zipper pull on it, which is inadequate. I don't want anything metal rubbing up against my horn's surface. No zippers, no zipper pulls, no snaps, no nothing. But as much protection as I can get.
Enter the neoprene personal electronics case. These are intended to provide protection for your cell phone, GPS, ipod, ipad, Galaxy tablet, etc., etc. The sizes are fairly standard, and you just need to know which one is likely to fit an alto neck, a tenor neck, or a mouthpiece.
Here is a neoprene pouch that is intended to fit a 7" tablet. Overall dimensions are about 8 inches by 5 inches. Perfect for an alto neck.