With just a bit more effort you can build a simple cold frame - essentially a low greenhouse with a translucent hinged top. Placed against a south-facing wall and recessed into the ground it will keep plants dormant but not frozen (unless you live in the colder regions of Canada, for example).
Dig a rectangle about 6 -8 inches deep beside the wall. Using scrap lumber or plywood, build a frame with sides the slope down toward the front; a 6 inch slope is sufficient. Make sure the front is high enough (about 18 inches) to accommodate your shortest bonsai. Then set the frame againt the wall and spread 3 - 4 inches of gravel in the bottom.
Traditionally, old window sashes formed the tops of cold frames, but you can also use clear plastic, fiberglass, or polyethylene plastic sheeting. In snowy areas, first cover the fame with fine mesh chicken wire or hardware cloth.
Place the bonsai on the gravel base and surround and cover the pots lightly with straw. Close the lid for protection from extreme cold, opening it slightly for ventilation when the temperature is above freezing.
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