With peak artwork unprintable & 1 week wasted, the lion kingdom switched to peak efficiency.

A simple cross would do the job in the least space.  It's barely visible, so artwork would be a waste of time.  It's true that the front corners have no support, but these speakers haven't had any diagonal falls.  

This design worked & was parametric, so changing the angle was just a matter of changing a number & printing out another one.  The ideal angle is between 25 & 28 deg.  It also depends on how high the seat is, how close the speakers are.