Please measure the circumference at the area you want the gear to be and be as precise as possible. As Leonardo Da Vinci already said, measure twice, 3d print once.
This is a little show case for our customizer software paramate and our script-based CAD system trCAD. We want to open our software to the community and are currently looking for beta-users. If you are not afraid of coding and would like to give it a try, get in touch!
The teeth of the follow fous gear have a module of 0.8, the industry standard.
The are several 3D printing services that will happily print it for you.