I got a Feasycom FSC-BP108B iBeacon to be able to track if my cat was inside or outside, but he freaked out having something flopping around on his harness.  I designed this to allow the beacon to be mounted flat. You can use the loop on the opposite end or (very small) zip ties anywhere along the center.

I've mad a rigid version that will work well with ABS/PETG/PLA or a flexible version with a lip that works with TPU.