Who's EFIS

You don't say what brand EFIS you intend to use. If you have a GRT system they sell a double stack AHRS for a dual Horizon system that will screw/bolt to that mount. I put that mount from Vans unnecessarily in my 9A intending to use a GRT Sport EFIS only to find out later that the AHRS is mounted into the units head for that model.
 
Haven't decided yet, looking for options. The G900 looks like it needs a lot of space, if the G1000 installations are any indication.

You don't say what brand EFIS you intend to use. If you have a GRT system they sell a double stack AHRS for a dual Horizon system that will screw/bolt to that mount. I put that mount from Vans unnecessarily in my 9A intending to use a GRT Sport EFIS only to find out later that the AHRS is mounted into the units head for that model.