[Cothron Theiss]

First off, what wheel diameter are you planning for? With 4" wheels, this is verrrrry slow, which may be what you want. Also, I would play around with Dillon Carey and Aren Hill's wonderful new design calculator to decide if you want to use a 3 CIM or a 2 CIM+MiniCIM configuration. (I'm a fan of 2 CIM+MiniCIM.)

How are you planning on making these plates? CNC mill, router, water/laser jet cutter? If it's water/laser jet, your fillet diameter should be fine.

You have stand-offs on your top two bolts; I would put these on your bottom four as well. This stiffens the plates and makes assembly quicker/easier since you don't have to worry about over-tightening those bottom screws.

Your lightening pattern looks very good to my mostly untrained eye, but I would run some simulation on that just to confirm. I will say that your mounting holes and bearing holes do not allow for much tolerance in your motor or gear spacing. In a perfect world, this is fine and in the real world, this is almost always fine. But if you want to be able to change gear ratios or motor combinations easily and often with this gearbox, you may want to add another set of CIM mounting holes with a slightly different spacing that are rotated around the shaft clearance hole you already have. Just some thoughts, you could probably leave it as-is and be fine.

Oh, and make sure you consider how you're going to assemble/disassemble this on the fly. You may want to tap the first plate (the one mounted to the frame), change the tapped holes in the second plate (the one the motors are attached to) to clearance holes, and use lock-nuts to fasten the second plate. This would let you pull off the second plate with the motors and at least one stage of the gearing, while leaving the rest of the gearbox mounted to the frame.

With relatively easy access to the gearbox and a quick pit crew, you could use this design to change your drive ratios between matches during competition.