For a simple protection for one or two small shrub sized trees, another idea might be to cover then put one or two weather proof utility light fixtures with incandescent bulbs under the cover.
I suspect you will need to experiment a little to determine the wattage light bulb needed, best placement, and whether the bottom of the cover needs to be snugly secured -- maybe with earth staples, lumber or bricks, buried edges, etc. And most importantly, if and when the light needs to be turned off and the cover needs to be removed next morning.
Some other members have mentioned using incandescent light bulbs in coldframes and under covers for freeze protection before. Off the top of my head, I think hendi_alex and pepperhead212 ....
Older incandescent Christmas lights can help in this way, too.... In fact, they can sometimes cause problems when wrapped around deciduous tree branches and left on them through the winter without due consideration because the trees don't properly go dormant.
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