Couldn't find a light aluminum L-Bracket at the hardware store, so I made this little plate from scrap. Basically drilled a few holes to mount it directly to the rotation head, then sacrificed a cheap sunpack monopod for the head. Seems very solid, and there's just enough room for the camera. Now if my intervalometer will just arrive!

I use a 90 deg monopod tilt mount on both my tracking units, but do have them mounted on L brackets.
It's really fast to level the horizon that way or make minor adjustments. Having a rapid connect plate there saves a lot of time too. I use the Manfrotto 234RC.
Given that they all generally tilt a little more than 90 deg, it is easy to make a flat adapter plate if you can't find an L bracket (as Cody has done).