If we add a mirror such that the frequency shifted beam is reflected directly back irrespective of the drive frequency, then the twice shifted beam will go exactly back to the origional incident beam on the AOM. See the attached sketch. A spherical mirror with radius of curvature equal to the distance to the end of the AOM should do this.

For our setup, the mirror is about 4 inches (100 mm) away from the AOM. Therefore, we should use a dialetric coated or silver coated (cheaper) mirror. Newport has worse options.

The addition of this mirror would change the mode matching solution. I think this change is worth it.