If you're looking for a standard size (i'd recommend keeping standard), I would start by downloading a model from Mc-Mastercar or TraceParts that will fit your needs. If you're looking for a custom solution I can send you to a link for the math on the design. They're actually pretty simple to model. If you use Solidworks I can look at posting an example of a model later this afternoon.

https://www.mcmaster.com/gear-racks
https://www.traceparts.com/en