This is the servo arm I designed based off of other designs that you can find all over the web. It's fits to an HS-55 servo you can pick up at any hobby shop for around 10$. The length of the servo arm is specific to the stock Mendelmax 2 with a J-Head. If you are using a E3D, or another extruder you may want to use a longer Servo Arm. You can attach an HS-55 servo with this mount https://www.youmagine.com/designs/x-carriage-hs-55-servo-mount-for-mendelmax-2-0