[Chris is me]

Comments on the lightening being too aggressive are bang on. Add some webs from the lower mounting holes to the outside of the plate, and a web or two on the bottom, at the very least. You're also missing a few fillets on your corners on the bottom.

The other comment I would add is that custom gearboxes are cheaper in terms of dollars, but the cost comes out of other things. If you are outsourcing parts to a sponsor, be very careful that you do not ask too much of your sponsor. They are donating machine time and operator time to your team - despite what they say there is a finite amount of this they may be willing to give, so be careful not to outsource more parts than necessary to generous sponsors. If it is a waterjet / laser sponsor, it's probably not much more time, but gearbox plates could be a few more hours on the CNC mill. For an offseason robot maybe skip the lightening to cut down on machine time?

If you're doing this in-house, none of that really applies. In either case, get it tested early and make sure you've worked out all the bugs of your custom gearbox. They can be a little tricky sometimes.

It's great to see you learning how to design robot parts - keep doing this. Keep posting your results, and be sure to work on your CAD in such a way that it allows for easy future iteration. Adam Heard's 973 RAMP videos are invaluable for this sort of thing.