Optimal opening size in media wall for Sonos Arc on extendable Sanis mount

Hello,

I would appreciate some advice please. 

I am mounting my arc on the extendable Sanus mount within a recess in my media wall. I have read that the Sanus mount sags a little under the weight of the arc and so I was wondering how high an opening I need to allow the arc to move in and out freely. 

Thanks very much for any advice. 
 

Nu