I reckon a standard stereo tie clip mic would be perfect. Get some half inch thick foam and a bit of fur if necessary and form a shape and use some gaffa tape around the edges to make it a snug push on fit. Or gaffa tape it on more securely and levee it attached. The mic is good at giving a natural sound of the environment you're in, and with it being a clip on mic you can easily adjust the balance of distance from your mouth and the tones, or nearer the mouth to reduce background road noise (if your gopro has a manual mic volume control. The tie clip mics are cheap, but reliable and work well. I used this as a mobile set up for a few years prior to zoom digital recorders, for live outdoor acoustic music. I've even used it as a vocal mic, on a harmonica rack through a small amp whilst busking. Good versatile and cheap as chips 😉
