OK, After a long stall, a pm response to this thread got me going again and these are DONEish. The only real stumbling blocks were choosing glue to attach the felt, and finding suitable hardware to replace what I was missing...and everything else I work on intermittently.
I chose Tacky glue (PVA) as I wanted something rather forgiving long term in case changes are needed. Using a toothpick I made sure every contact surface was covered and attached the felt. I also used the same method and glue to attach the perforated metal protective grill to the plastic capsule frame. There's a really thin edge here, but it's doable (and advisable to do before installing the capsule). I also cut a circle of felt to cover the back of the capsule, as there appears to be something similar in photos of the real M380. A few small dabs of glue to hold in in place.
Regarding hardware, I actually remembered to take a screw with me to the "hardware store" which, as it turns out, had matching screws. In the "Specialty Fasteners" section I found 2 packs of M2.5-.45 x 6mm machine screws which fit the mounting holes, both for the internal shock mounts, and the two holes at the top of the body which hold it together (though those are countersunk and I already had the originals). The hardware that holds the capsule frame to the shock mounts is a screw and nut, so I just dug around and found some that would fit and used those instead of bothering with an exact match.
In the dead TG-X50 I git, the capsule frame has a strip of foam tape, like weatherstripping, sort of, on the outer edges. It looks to be about 1/8" thick by 1/4" wide, not an off the shelf size I don't think but I had some 1/4"x 3/8" weatherstripping on hand so I used that, for now anyway. It's a sort of tan color and since it's too big, it conflicts a bit with the foam windscreen, but not enough to be a deal breaker.
So once everything was otherwise assembled, all that was left was to solder and close them up. The coils on these capsules are super fine wire, and too much heat from the iron along with some foolishly applied pressure meant I popped one of the solder tabs completely off and broke the connection to the coil. I was able to locate the end of the wire and successfully re-solder it, but don't let that fool you into any sort of security. That wire is so fine it's nearly invisible and I thought I was going to melt it altogether when soldering. Frankly I'm amazed it worked. Just turn that heat down a bit and be really careful if you attempt this. Also, it's worth mentioning here that the real M-380 has a humbucking coil, which I can't find any useful info on, and the thread I referenced in the OP omitted it and noted no significant difference in performance, so I've gone ahead without it here as well.
Once I got everything soldered and confirmed connection, I stuffed a but of felt left from the headphones into the neck of the bodies as shown in other pics, and closed them up. And...They work! I don't have a real M-380 available for comparison, but they have an obvious fig-8 pickup pattern and a proximity effect that seems to correspond to descriptions out there. Oh yeah, I made a pair of these, hence all the plurals. I'll throw in some pics below.
This is totally doable, but I realize getting the bodies is a huge setback and I'm lucky to have found some. I'd think with some more serious physical hacking one could adapt the cans from the headphones to work for this, and the open-back models like the DT-880 would probably be better than the closed DT-770's. But otherwise the only difference would be shape, and a stand mount would need to be worked out, but it could totally be done.
I may still change the foam stripping to the right size if I can find it, and I have a sheet of foam I may use to cut some fresh pieces fort he grills.
Finished product
Inside rear, Attaching felt
Screws (philips are new slotted original)