simplest connection would be the appropriate spline with a flat
plate welded on its base. To avoid binding, we would need to build
our motor and differential mounts to a very fine tolerance. In addition,
we would have to make sure the motor and differential did not move
in relationship to each other under our torque loads.
solution is to plan for a little slop and use a Constant Velocity
(CV) joint to allow it. CV joints are usually used between the differential
and the wheel, to allow torque to be transmitted to the wheel while
the suspension is moving. The movements we expect between motor
and differential is tiny compared to a wheel going over a bump.