This is a couple of different flavors of extension pieces that fit on a PS3 Controller Thumbstick. It is based on the shape profile in the http://www.thingiverse.com/thing:10837 by scooter46290.

I have printed and used both of the Ext2 and Ext3 shapes and they are wonderful to use. They fit on the thumbstick nice and snug. The convex or concave shape is a user preference. I could not believe the impact these things have on FPS gameplay. I normally used to play MW3 with a look sensitivity of 6 and now I'm playing on 10 with one of these on the right thumbstick!!! I am only using one on the right and normal on the left. There doesn't seem to be much point for me to use an extension on the left, but that's just my opinion.

I printed on my Mendel.

A little light sanding on the thumb pad of the extension with 100grit sandpaper makes for a nice texture for your thumb to grab onto.