If you are serious, the answer is to make the device a 1" tube that fits into the steer tube of most bicycles and can receive and transmit RF via the top cap (which would be plastic rather than the usual metal). Easy to conceal, near-universal fit (for bikes from the last 10+ years), sufficient space for a battery, radio, no visible parts. Charge using the tube as ground and the top cap bolt as hot.

The problem is, it will cost $150 or so, plus monthly cellular data fees (you can dial 911 for free, but the police will not do anything with this).