Ricky,
Yikes that would not be fun.[]
I have the exact same size boots and mine has never slipped out from my feet? I just make sure that the strap is right over the " Crease" in the top of my boots with the bungee cords pulled up completely over the entire heel to my ankle.
Also pull with your left hand to get the strap tight.....you need to pull across your feet, not upwards with your right hand.
And don' t start climbing unless you can feel that the straps are tight. They should be very snug on the tops of your boots.
I have had to really wiggle my feet sometimes to get the platform loose while climbing and even then my feet have not come out.

Good luck, and you might want to shorten that tether line up some while you work out the bugs.