This was pretty straightforward. The only R13 insulation I could find at Home Depot was faced (has the brown paper backing on it), but that made it really easy to install. For the back, side, and top, I glued the paper backing to the inside of the speaker box. Pretty simple. For the insulation on the cross brace, I removed the paper, so as not to interfere with the absorption of the insulation.
I installed the insulation before I glued on the top of the speaker box. It made access very easy.
Insulation on the back and side.
Insulation sitting on the cross brace.
And then I glued the insulation to the top before I glued the top onto the box.
Next up, the 2nd baffle, and cutting those curved corners for the waveform. I was able to successfully do it on a test piece. We'll see how it works when I do it for real.