Idea to protect small perennial trees in unheated greenhouse

I have a small plastic covered greenhouse and have started some small seedling jujube and hardy kiwi.  I want to wait until the end of next summer to plant in ground.

I made the 5 gallon buckets into an Air Prune design and am worried that during the real cold weather (low of 9F for 3 to 4 nights) the low temperature would kill the roots.

I came up with this solution (design attached).  A vinyl sheet 4 sided box would be used to hold straw around the buckets over winter.  Since it will lay directly on the ground (on ground cloth), I am hoping ground heat will help along with the straw insulation/heat.

I may also cover the top with frost cloth or bubble wrap (or both).

Please comment and let me know what you think.  Should this work?  Any improvements?

Thanks