Here are a couple of threads to look through if you haven't already which might give you some ideas. Basjoo is pretty easy to protect, and I wouldn't worry too much about watering, I never water them over the winter. If you have a local landscape supply center and can get a hold of some inexpensive mulch, pile it up around and on top of the plant and box, you should be fine, If you want to protect the above ground growth, its pretty easy to to do and you don't need to overthink it. Even just a garbage can over the main stem (I usually only protect the one stem) should work in your zone, but I do a little more than that as the pics in the following threads show.
Uncovering winterized bananas
The last page or two in this thread shows my process, but again, no need to overthink it. I know a lot of people that just pile on some mulch on top of the mat and call it good
Time to put the bananas to sleep for the Winter