Cut the coupler down so it's 2" long. It doesn't matter from which side you cut it.

Get the shaker motor, the mount you made, the shaft/hammer, and the coupler.

Put the shaker motor onto the mount and couple the shaft to it. Mark where the shaft enters the coupler.

Make sure the coupler won't be rubbing on the mount while it spins.

Make sure the hammer has about 5/8" clearance from the mount.

If not, you can pull it out of the coupler a little, but make sure the bolt won't be too close to the edge.

Otherwise, you can grind away the tacks on the hammer and tack it properly.