There are quite a few variations of the expandable card wallets on the market… These I guess, are the thinnest wallets you can have and they mostly consist of two plates or fronts (of various materials) held together by rubber / silicone o-rings or an elastic band.
Slimtech pictured above has a very simple carbon fiber or aluminum design. You have an option to add a carbon fiber strap for storing banknotes and you may brighten it up a bit with tension straps of various colors.
Elephant Wallet have two models of this type: N wallet and X wallet of which both also offer an option to choose between carbon or aluminum plates, held together by an elastic band on the outside of the wallet fronts.Third one I would like to mention here is a bit more sophisticated and also held together by an elastic band, but in this case elastic is hydden under the plates. It’s called The Ridge wallet and you can choose between aluminum, titanium or carbon fiber fronts. I would say this is my favourite of the three.