OK, first to answer questions, then post a little progress...
Adam - In retrospect, I think I would have built this a little bit differently, just to get more insulation in there. Your question makes me think that you built yours such that the frame was outside the width of your fridge. I kind of wish I would have thought to do that. If you look at the earlier pictures in this thread, you'll see that the outside of my frame is just about even with the outside of the fridge. As a result, instead of 1.5" of deadspace in the frame and then insulation inside of that, I have 1" of blue styrofoam insulation tucked into the space inside the frame members, and .5" of dead space. There's also an extra layer of this along the bottom - it was done mostly to bring the inside of the chamber level with the inside of the fridge, but it has the added benefit of an extra R5 on the surface my fermenters will be sitting on. This stuff has an R value of about 5 (of course, reduced a little by the sections of 2x4 interlaced in there). Then, inside that, I have 1" of foil-backed rigid fiberglass - roughly R7.
If I had it to do again, I'd do one or both of two things:
1) Frame with 2x3's to ensure more room for insulation and less wood.
2) Frame it out such that the frame stayed outside the width of the fridge so that I could get in more insulation.
Can't yet answer about the coldest setting... I left it set to 6 of 7 overnight, the mason jar hit 44 and the better bottle is lagging a bit behind at about 46. I just made one of my last tweaks to the fermenter and cranked it up to 7. Gonna let that run about 24 hours, see what happens, then tomorrow I shut it down, empty it out, and prepare to start priming and painting.
So, the picture, which I think speaks for the change I made pretty effectively:
I cut holes in both sides of the box and installed vents to let some of the heat escape more rapidly from the sides of the fridge. Pretty simple tweak, and I hope it's effective. The vents were big-assed soffit type vents - 16"x8" would be a BIG soffit! But hopefully they work for my purposes.
Lastly, a shot inside the box, just to give an idea of scale. And don't be alarmed by the Bud case in there!
A buddy gave me a bunch of bottles in said case, and right now that case is full of an American amber ale that I'm waiting for another buddy to show up and pick up.