(I messed up with one of the holes, which is why the wall piece on the right has three holes on top. But the extraneous hole will be covered by the connector anyways, so the aesthetics won't be compromised. =])
I also finished making the back wall. The rectangular cutout is for the ports on the Arduino.
Finally, I cut out some of the L-shaped connectors:
Tomorrow I will drill holes into them. This is the last part of my project that I have to machine, so after that, all that remains is the assembly (the fun part!).