The ultimate lazy approach. Zip tip or velcro this.
https://www.amazon.com/Universal-Spe...ph+speedometer
and power it with one of these.
https://www.ebay.com/itm/Waterproof-...72.m2749.l2649
Or get said USB charger above, buy a Ram Mount, and rock a GPS Speedometer app from your cell phone. I have a friend who does that on his old bike lol.