Summary from the designer: 

Upgrade those TI Computer Joysticks!

Avoid the hand cramps! I designed and upgraded my joysticks a number of months ago and they are now my preferred joystick on the TI-99/4A (over my Atari and Wico joysticks)!

There is a discussion link on AtariAge:

https://forums.atariage.com/topic/338357-revisiting-the-original-ti-99-joysticks/

This is just the 3D printed parts. Switches and other wiring and hardware are not included.