Finally, some progress pics. Round two of SLS prototypes has worked out well, and I am ready to get these suckers made up in metal. The fit is nearly perfect. At this point, inconsistencies in the castings of the FX-R projectors cause more variance in things than the adapters, so it's time to call it "good enough".
One big item I wanted to make sure of was the clearance between the projector lens and the headlight cover. To do this, I used some painter's putty since it doesn't contain abrasives.
*squish*

The thinnest part is 1.5mm, which is what was predicted from my measurements of the parts.

There is also 0.5-1.0mm of clearance between the lens retainer ring and the decorative chrome "smiley" cover. It is a bit warped, probably from decades of heat cycling, so it may take a little tweaking when I glue everything into place eventually.

Here is the projector attached to the 2nd generation SLS (3D print) prototype.

"Inside" of the adapter.

"Outside" of the adapter.

But bmwman, isn't that thing like, overly complex? What's with all the intricate nonsense? Well, it so happens that the price the 3D print house charges is directly proportional to the actual volume of the part. So, I modeled my adapter and then cored the hell out of it. They say that they can do a minimum wall thickness of 0.7mm, so I went with 1.4mm since you just aren't going to get consistent 3D print results when you are riding the limits. Warp, sag and undesired holes become issues when you push the wall thickness limit.
A solid one would have cost $37, and these were $23. Not bad for an extra 20 minutes of work.

These SLS parts are super strong. I didn't have an M4x0.7 tap handy, so I just drove the machine screws right into it. The part took the threads fine without splitting or anything. They are flexible and strong (surprisingly so). The material is nylon (sintered powder).
My next steps are to make a couple of final tweaks and then proceed to make (or maybe have a fab shop do it) the aluminum adapters. From there, I will be buying new bucket adjuster screws since the ones on these were shot, and then assembling + aiming the projectors. Since they are NOT sealed, I will be looking at my clearances in the headlight brackets and seeing about developing rubber boots to keep water and dust out of them. Worst case, I just use some aluminum tape to close things up, but I think that I can come up with something more OEM looking. I'll post up a concept / cartoon of what I am thinking of in a day or so.