I modified my gearbox plate to look more like yours and did a simple factor of safety analysis. A lower factor of safety corresponds to more load on the material, and a greater deformation of the part.
You could likely get away with what you have drawn up, but some simple improvements will yield huge strength increases. The biggest change you should make is using the 0.75" boss on the front of the CIM to help locate the CIM. It'll increase the positional accuracy of the CIM by quite a bit, and it'll make the part even stronger.
All tests were done with the the mounting holes as fixed geometry, and the only load was gravity acting on the 2.82 pound CIM motor. You can roughly double the strength of the part while adding about 0.03 pounds.
You can also see why having a convex outline helps too.
http://imgur.com/a/xTIpW
This is what we came up with last year for a 3 CIM configuration:
http://imgur.com/DSjie1M