With the gearbox requiring some TLC and a wire brush and maybe some WD-40, I quickly traced the outline onto some stock wood to cut a template.
As you can see from the second pic, I'm going to have to cut a space for the drive shafts to fit. I don't think it's necessary to be 100% accurate, since the adapter plate doesn't need to be watertight, but I'd like to get it as close as possible, just to eliminate the possibility of vibration or problems in the future.
You can also see the dashed lines at the "top" of the adapter plate. Rather than making the top match the curve of the gearbox, I'm thinking of extending it up and having a flat top with a series of holes drilled into it. This would make it like a modified Picatinny Rail and enable me to bolt a shelf to the top of the adapter plate to hold the controller or support the battery rack.
The good news is that now I can start talking to my local machine shop/fabricators and actually have something they can see, measure and work with, rather than just drawings on paper. YAY!