Making your own controllers using the Microsoft Adaptive Controller. Focus on DIY and COTS re-use.
Newest (working) code is HandheldUSBJoystick_ForXBOXAC_V1
The joystick library I used should be the standard one. If you need a copy, grab joystick.zip and put in /libraries/
HandheldUSBJoystick is for "Playstation 2" style sticks you find cheaply on Amazon.
JoyConStick_Converter_ is similar -- except it's coded for JoyCon Style Replacement Sticks (not the whole JoyCon -- just the stick and breakout board). The JoyCon sticks present different / odd ranges, so this code handles that.