It seems there's no agreement among hobby servo manufacturers on what exact dimensions their servo horns should be. You'd think it would be the easiest thing in the world for them to all make their horns according to an agreed upon standard, but apparently this is not possible for some reason.

This state of things has presented me with a problem, since the two 9g servos that drives Tardygrade needs to fit their horns into precisely dimensioned slots in the 3d printed gears.

To remedy this I made two parametric OpenSCAD scripts that lets you customize the gear parts that slots the servo horns. One simply need to take some measurements of the horn with a caliper and enter them into the script. Then the resulting stl should be a perfect fit for that particular servo horn. 

I used FreeCad to model the the gears. These models are then imported into the OpenSCAD script, which is only responsible for making the customized cutout for the servo horn.

 Yep. Works pretty well.