This is indeed some impressive work, but/and seems to me that it only supplies a concealment strategy for one make of instrument and four models from that maker (86b 86c 87 88).
Without criticizing any of this marvelous work, the fact above - combined with me practically being stuck in the bed for quite a few days - has prompted me to think further.
I’m now conceiving of a fake strap ring soldered to the back of the instrument – perhaps at the center of the bottom bow on the back side…
…and sure: it could replace an existing strap ring.
To any voodoo-embracers (ie. “affects the sound”), I don’t believe that I would hesitate to do this to any of my instruments, none of my instruments are for sale, and all are precious to me. Obviously, the soldering would need to be done very nicely.
I believe I would start with a hollow brass tube with at least a .100” wall thickness and (??) something like (prior to custom fitting to the instrument) 3/16” taller than the Apple device. One end of that tube would be installer-ground in on two sides to imitate the curvature of the bottom bow, The other end would be inside-threaded to accept a (ok: 1/8” thick) fine-threaded round lid, and a low profile strap ring could serve as a wing screw for a round threaded lid concealing the hollow space.
It would need to be well-made enough so as the lid (with the strap ring brazed to said lid) screwed down flush with the sides and with absolutely no visual traces of threading.
I’m not going to defend this idea has any sort of be-all/end-all, but I think it’s a good idea, and it’s a good idea to keep considering ways to accomplish the primary goal.
That having been said, I do believe that my idea represents a bit more simplicity well as at least as much believability - in regards to “looking like something that belongs on the instrument“.
Just be aware that any of the Casanova tuba players out there can’t be two-timing spouses/significant others (between-rehearsal romances, etc.) with something like this tracking device on board.
