As you can see in the attached pictures, I'm using a Meccano type of construction set to build the robot. The images are part of an arm that is mounted on a vehicle. The construction set is called Merkur and it's important to note, that it's using metric for the spacing of the holes, rather than inches and it matches the mounting holes on the servos.
What I have done is, that I have taken the bottom off the servos and drilled a hole to mount a screw at the bottom. The screw and the center of the servo horn forms a vertical "axis" through the servo and the servo rotates around this axis when it is turning. The point is, that I can mount the arm at both the servo horn and the bottom to make a more robust construction. Please note, that not all servos have space at the bottom to mount screws. I'm using hexTronik HX12K.