I did a very similar thing with the keycard to my workplace a while ago. It read fine even through the metal of the stock iPhone back. However, though the back seemed to close fine on the pieces of the keycard, it caused the glass to bow slightly and the glass eventually broke when the phone suffered a light impact. I’m going to try again with a custom wound antenna and Kapton tape.
Make a robot friend with Adafruit’s CRICKIT – A Creative Robotics & Interactive Construction Kit. It’s an add-on to our popular Circuit Playground Express, FEATHER and other platforms to make and program robots with CircuitPython, MakeCode, and Arduino. Start controlling motors, servos, solenoids. You also get signal pins, capacitive touch sensors, a NeoPixel driver and amplified speaker output. It complements & extends your boards so you can still use all the goodies on the microcontroller, now you have a robotics playground as well.