Absorbing Sound Reflection From Side Walls in a Rectangular Room

I've been working to make my existing mixing setup sound good acoustically rather than getting new pieces of gear. So, I decided to take it upon myself and construct my own sound panels for the side walls of my room:

The first thing I wanted to do was absorb the upper mid and high frequencies that reflect off the side walls. Why the side walls? The music I'm listening to from my monitors goes straight to my ears but when the side walls don't have any absorbtion, the direct sound gets combined with the reflections. As you might be able to guess, my monitoring room is a rectangle, which is exactly what you don't want. This reflection can color mixes negatively, so I started treating the side walls with some old Auralex foam panels, some cedar slats, partical board, and some glue.

Here's the general progression of how I put these together