I’m working on modeling a drivetrain in my free time. Due to the way things were designed, it needs a custom gearbox. I have never designed a custom gearbox, so I was wondering if anyone had any tips/tricks they would be willing to share 
First off, I’d recommend you post the designs here after you finish. It’s much easier to offer critique and suggestions when we can see the design itself.
-As always, add .003-.005" to the center distances of your gears.
-While you’re designing, consider how you would replace a motor, gear, shaft, bearing, etc., were one to fail.
-Don’t put snap rings or e-clips between loads.
-Consider how you’re manufacturing the gearbox before you design it. A gearbox to be made on a CNC mill should probably look different from a gearbox to be waterjetted.
-Look around on here. There are some beautiful gearbox designs that people have posted here.
-Don’t be afraid to use something other than the .5" hex bore 20DP standard Vex gears for your first reduction. The 20DP gears are a go-to for many reasons, but they can also be a little overkill for the high speed, low torque stage. Consider a smaller pitch gear, or belts, or something more creative. But make sure you justify stepping away from the standard, and back it up with solid engineering to make sure you aren’t cutting corners.
-When you’re making the CAD model, build it with the expectation that you’re going to change it. If editing a dimension or placement of a feature takes longer than it did to originally correct it, than you probably need to work on your CAD skills. Team 973 has some great videos on improving this if you’re using SolidWorks.
If you have any other questions, let me know. Good luck!