Ooh there might be something. Been trying to hide an AirTag on my bicycle and I am not keen on the bottle cage/saddle mount solutions currently doing the rounds. BB/Cranks are hollow and I’m convinced can be used to secure one as a pseudo crank cap. 

 if you got an .stl then no problem
printables.com
thingiverse.com 

 I’ll have to look into CAS/stl - skills/knowledge are weak in that area but will make it a little project over Christmas. 🤙🏼