This Joycon Knuckles is designed by Tokytome linked below. That's a great design, me and my friends like it so much!
I have slightly modified it by adding a slot as shown in the second photo, allowing the joycon to "snap" in the knuckle when putting it in. So you will need to press the small button on the joycon in order to release it, just like what you do when you release it from the Switch console. This can prevent the joycon from slipping out.
Also I found it can be printed without support when it is in upright position (third photo for your reference). It will need some bridging, but not very hard to print.
This is my first remix, hope you like it. And thanks again for Tokytome for such a nice design!
Octolapse - timelapse of 3D printing this product