I’ve posted earlier about reTyre who custom made a studded skin for my 36’er: Studded tyre for 36'er - possible solution?
reTyre have developed a new system were the zipper is sewed on to the tyre (like they did for me), and you can switch between asphalt, terrain or studded tyre in a minute. The retyre comes in the sizes 26, 27.5 and 29. The tyre width with the skin is 2.1-2.25.
If anyone’s interested, there’s a kickstarter campaign going on right now: https://www.kickstarter.com/projects/870651719/retyre-one-the-worlds-first-zip-on-bicycle-tire-sy?ref=Fundedtoday&utm_medium=referral&utm_source=webe.fnd.to