Thumbstick cap

The centerline allows the bending motion of our thumb joint as your thumb role from one side to the other and act as a hinge (make sure it is oriented orthogonal to your thumb when holding the controller). 

The elevated sides keep your thumb centered. The increased height (lever) also increase precision. 

in case it is loose use double sided adhesive tape (vhb or foam), hot glue or silicone to fixate if it isn't snug enough (you can use normal glue but this will most likely be unremovable without damage). I printed the low version in TPU which attaches firmly without any adhesive.