I came across a model for an octagonal stick insert and thought it was a great idea, so I decided to finish and improve it. I wanted an insert that was inconspicuous, allowed for full deflection of the sticks, and wasn't a permanent solution so I could go back to regular sticks if I wanted.

The part slides over the gimbals of the controller on the inside of the controller itself and restricts the range of motion of the backside of the stick so it behaves and feels like an octagonal stick. I included an assortment of tolerances so you can pick one that is tight enough to not slide around but not so tight that you have a hard time taking it off. Assembling the controller after installing is pretty straight forward, just pay attention to the left stick as there is a part of the top cover of the controller that will need to slide underneath a lip on the printed part.