I never have my Square Reader on me when I need it ... until I designed this Square Reader key chain!
Snap in the Reader with the Square Logo face up. It might be a tight fit at first but will loosen over time. Sand the side tabs as needed.
This is built for the current generation of Credit Card Swipers, which is the 2nd gen if I'm not mistaken. Those are the ones that measure 1 1/16" by 1 1/16" by 3/8".
The design has accurate dimensions, which means you may need to use "Horizontal Expansion" or "Print Outer Layer first" to ensure a proper sizing. In Cura, I find that -0.1 Horizontal Expansion prints it accurate to size.
To keep the keyring portion strong, print with 35 percent infill or higher.
I am also including the Sketchup file in case you want to remix this.