No need to make an expansion card mod and waste the slot. A stripped down airtag in the expansion bay works just fine, assuming you don’t use the graphics module.
Saw a forum post the other day where someone did the exact same thing, but I can’t find it. Wanted to lend credit to the mod because yeah it works. Unsure how this affects thermals in the long term so keep that in mind.
I removed the airtag speaker so assuming a thief isn’t a tech enthusiast, I doubt one would be able to find and disable it. Renders the device useless outside of a faraday cage lol