I removed both assemblies, and measured the passenger's side assembly with a cheap gauge and guesstimated what I couldn't measure. I modeled the section in Autodesk Fusion for Mac and uploaded the model to Shapeways.
A week or so later the part showed up and it was perfect! I guess it shouldn't come as such a surprise, but It's always exciting I guess.
I took a small hardware knife and cut out the bits I didn't need, then refined it with some 100 grit sandpaper. The filing tool I had wasn't quite rough enough.
Once the fit was right, I beaded some multi-purpose cement around the contact area and sha-bang! Good as new!
The last step was to find some thin metal to recreate the retainer clip. I rummaged around until I found one of those old 3 1/4 floppy disk shields I had inexplicably laying around. It wasn't quite as strong as the original, but it was close. The biggest trick I ran into with this was that there was a small punch-out in middle of the clip on the back, which catches a groove in the slot so when the clip is inserted into the slot it catches and holds the clip in place. Finally I was able to punch it out with a acrylic cutter which had a very small, sharp tip on it. it wasn't clean, but it worked.
I snapped it in and it survived its maiden voyage, which, is only a few miles and I barely hit 60mph, but usually it's shimmied part or all of the way out even then. I had discovered it was speed more than bumps that would pull it out of it's socket and leave it dangling by its twist-on boot, so even if it keeps me from having to put it back in place every couple days (or having to re-apply tape every so often) it's pretty much a win.
All-in-all, it came out pretty great and saved me probably twenty-five or thirty bucks. Was it worth it? I don't know, maybe not, but it was fun and makes for a pretty cool story. Hope this inspires some more car repair! And if you've had the same problem, the part can be found over at Shapeways. Feel free to print it off and fix your 1990 E30! Should work for all four markers!