Cold Frame Plan
After sitting in on John B.’s November Bonsai 101 discussion on wintering, I recalled getting this PDF from a gentleman used to winter a couple of his more sensitive trees. He actually wintered a nice-looking Japanese Trident Maple and some apple/malus variety in this which was in his protected backyard in Markham, ON. Of course he had dirt in this, which he dug the trees into like we would normally.
It is relatively portable for folks with limited space. The article also touches on window-well storage, as John also touched on.