I have a “Top Trek” stem/handlebar mounted bag that has a transparent cover where the phone is securely held by Velcro flaps from the rear of the cover. It allows limited interaction with the phone (which I don’t care much about), but allows the ‘bike mileage app’ to show very nicely. The bag itself has room for a spare tube, tire levers, patch kit and multi tool.
It was on Ebay for about $12.