I would take 6 plates, with a bolt in the middle, weld the head of the bolt to the plate, then drill 6 hole's in the side of the BII, 3 down each side of the back window, then get three heavy duty bars, and bend the ends down, then weld to the 6 plate, one bar on two plates, this way they are bolted to the side, and with the ends turn into the plates, it won't get stuck in to the tree that it rubs.
I don't have auto cad right now, or I would draw it out for you to get what I'm typing about.
Robert