I had bought a couple "thin" wallets on Amazon, but none of them were exactly what I wanted, so I designed my own. This makes use of wide rubber "ranger" bands available on Amazon here:

https://www.amazon.com/dp/B07DKRP2GY

3.5" x .75" bands ($8.50 as of 11/2019, 15 count, so you have replacements or can make a bunch of these).

There's a "bump out" on one side to hold cash better. There are slight bevels on each side to more easily insert cards.

I've been using mine for 2 months now and it's been working perfectly for me.