EVT Safe Zone Bicycle Helmet Mirror https://www.amazon.com/dp/B003LVHZ4O/ref=cm_sw_r_cp_api_i_110W8T8H1TE99CK337QG

Stupid that it’s $40 bucks, but I guess you get what you pay for. I’ve tried just about all the cheaper options on Amazon. This one is by far the best for me. Fixes to the helmet so you don’t have to spend time finding reference points each time (like with a hand mirror). Strong enough and flexible arm to get the thing where you want it, and it’ll hold there in the wind.

Attaching it to a motorcycle helmet will be tricky since they’re all slick on the outside. Maybe you’ll find a way to be creative, or find a adv/dirt helmet you don’t mind messing up a bit.