I would probably position it by contact cementing it to a small 4'-long stick on top, applying superglue to the bottom, positioning it, allowing the superglue to dry, and then pulling the (small/long) stick - with the contact cement on it - loose.
That thing's way smaller than the dents I see in most pictures of most all not-brand-spankin'-new tubas,
When the battery dies (or new much smaller/better/stronger/cheaper airtag is developed), the same stick (or flat metal) should be able to pop that superglue loose in a couple of seconds.
Before ever gluing it in place, I would position it, and determine/test whether the airtag's webpage picks it up.
I would predict (based on advertisers' past research) that it would raise both the 3rd and 5th partial pitches, promote a world-class sound and - otherwise - offer no ill effects on the playing characteristics.
bloke "Otherwise (come on, folks - It's just a 641) who would steal one of those, anyway...??"