ROCKBROS Bike Helmet Mirror 5.5cm HD Mirror, Flexible Frame, Detachable Design, MTB Helmet Mirror for Bicycle Cycling Helmet, Bike Accessoires

https://www.amazon.com/dp/B0DB7QKG9Q - I really dig this helmet mirror. It's bigger than most and extra dorky, but it works great. I never felt like I could see anything well on the bar end mirrors. This one gives a great line of sight. The ball joint on it gets a little loose over time but you can shove a tiny piece of rubber band in it to tighten it up.