This is my take on a shell for Nintendo's Joy con's. IT works the same way you're stock on does, You'll slide the controller on each side and you'll need to press the button on the back to release them.

I printed the shell and the two slides and then CA glued the slide inside the shell.

I'll update this file to include screw holes when I get the change which will allow you to use the slides that came with your shell in the box, I've just been so busy I haven't had the time to get it done.

Note, the photo may not show the perfect fit around the bottom curves, they are updated now, and should fit much better.