I believe the two most popular places for the Roll Servo are in the wing and under the floor, to the right of the front seat. Some don't like the under floor option because of interference with the backseaters footwells (if you have those installed). I was able to make the geometry work and still have the footwells, just takes a bit of fiddling. Pitch servo is pretty standard. I basically copied some pics I had saw on VAF and other places of pitch servo mount designs and made my own. Dynon AP install manual had a good diagram/plan. Rigidity is the key. These are GRT servo's but I believe the Garmin's are the same form factor or close to it.
disclaimer: The roll servo pic is not the final install... the geometry is better now and has been working for 150 hours.
Also, I'm not sure why the last pic is upside down. It's right side up on my computer but flipped when uploaded to VAF.