Mike, are you getting wind and/or rain with the cold? Is the cold be sustained or will it dip down for an hour or two?
I do not like the idea of a black plastic bag, especially if your trees are newly planted. The bag will act as a huge sail which could pull on the roots. If you can erect a quick frame around the tree (using stakes or wire mesh, etc.) then drape a sheet or blanket over the frame, it might help increase the temp immediately around the trees. If your containers are all together, the potting mix will hold on to the heat it absorbs during the day. Again, you can drop a sheet over them.