If it were me, I would just incorporate some "scary behind stuff" into training.
Get your mares used to trusting that you won't let anything scary attack them from behind. It will take a lot of work and not everything will get covered.
You could start with things you know they aren't afraid of, and make noise with it behind them (far enough that no one gets kicked!) and then let them go investigate what that noise was. Do that over and over again and eventually they'll be less spooky. Hopefully anyway!