This evening I attached R2's back door, thereby closing the tin can that is my droid.
First I taped the back door as tightly to the skins as possible, centered in the proper position. Then I drilled four holes through the skins, and through the brackets behind the skins, using a #43 drill bit for tapping a #4-40 screw. Two holes are on the left side of the door, and two are on the right side.
Next, I tapped the holes in the brackets with a #4-40 tap.
After that, I took the back door to the drill press, and widened the holes with a slightly larger drill bit that allows the #4 screw to pass through. Note that I still need to countersink these holes.
Finally, I did a test fit of the door, with the rear power coupler loosely fitted at the center-bottom. The door fits pretty well, except the top still bows out a little bit from the body. :(
I'm not entirely sure how I'm going to address the problem of the top of the door not sitting flush against the body. I may add another screw or two at the top, or I may just live with it, as it's not that bad. I also managed to knock out both of my rear coin returns when I was trying to bend the door to fit the body. I'm not too heartbroken about this, since I have a set of Wayne's coin returns on order, and I may just use those instead. For now, mine are taped back in place.