PCB module is about the size of a nickel and has an ARM processor and a sensor chip or two, plus a connector with 5 or 6 wires.
Module must be firmly attached to device because device will be vibrating. Environment of device may vary: hot/cold, wet/dry, wind/dust.
Cannot modify the target device in any way.
All I can think of is epoxy the bottom (non-circuit side) of PCB to device, which will be metal or plastic, possibly painted. Specifically, coat entire bottom of PCB with epoxy and stick to device.
Surface of device is flat. Device surface may be larger or smaller than module diameter, but always big enough for epoxy to work.
If surface of device is conductive, then standoffs would be epoxied between PCB and device surface.
I cannot think of another method to attach module.