Are you trying to keep the D shaft on the temporary motor untouched? If NOT, I'd have a machine shop cut a keyway in that temporary motor shaft opposite the D flat. Then have them make a piece that would fill the open D area where the temporary shaft and the sprocket come together. Finally, a dimple on the insert piece that your set screw would mate into.
If you don't want to cut a keyway in the temporary motor shaft, a keyway could be cut into the piece you have made to fill the D area. Depending on how thick that piece is, the D piece may split at the keyway when high motor torque is supplied, but even if it cracked, it should stay in place if you make the fill piece to tight tolerances. If you use this solution I'd make it so tight I had to drive the key and the D shaped fill in piece on to the shaft. A snug interference fit.
My choice would be the first case. If you had to use the temp motor shaft for a matching sprocket, an open keyway opposite the D flat wouldn't cause any problems.
