Inspired by Snortimer's Wall Bracket for TRENDnet Gigabit Switch, I decided to create a similar design in Fusion 360 and make it parametric enough to use for attaching various boxes to the wall.

The specific STL included here is probably not that useful, you'll want to open up the Fusion 360 model and change the parameters to fit your needs.

I also included a middle bracket for the case when two things are next to each other.