GMU-11
From the Garmin G3X and Touch Installation Manual Page B-2
"The typical GMU magnetometer mounting location in RV-7/9 aircraft is in the aft fuselage on the aft deck (F-714) just ahead of the horizontal stabilizer and just aft of the F-709 bulkhead. This location provides a level pitch and roll reference, and acceptable separation from excessive magnetic disturbances in typical RV-7/9 aircraft."
The GMU-11 can be mounted with the connector forward, aft, left or right. I mounted mine with the connector to the left and there is sufficient room on the aft deck for the four mounting screws without the need for an additional bracket. The G3X must be calibrated for the appropriate connector location of the GMU 11