The best ones available IMO, are by Vaja (
www.vajacases.com). The Nuova Pelle and the La Pelle. The nuova is a single piece and really thin, the la pelle is two thin pieces adhered together I believe. The nuova is a bit flimsier but deff thinner. I personally prefer these types of cases instead of the ones that have a snap on case built into them. The surface pad by twelve south is super thin but has peeling issues and once u remove it it's basically ruined.
I did a review of the la pelle here
https://forums.macrumors.com/threads/1652122/