After some further looking at this, not sure it does really need 4 wheels which brought up another idea for a design change. Since only 3 wheels are used, there is really no need for the plastic going all the way to the corners that I can see. Plastic volume is 3.6 less cubic centimeters (44.9 instead of48.5). I also fixed my support system for underneath. It was a little too close to center side.
X-MotorMountOnePiece_rev27.JPG