Normally, I wouldn't worry about a Meyer Lemon, even in a pot but yours are not mature trees. The easiest solution would be to put a kerosene lamp in each pot by the tree. If not a lamp, a halogen lightbulb. You need something that will produce a little heat, it doesn't take much.
If all else fails, start dragging but DO NOT cover them in plastic. An old sheet is better. They would most likely be okay where they are with a sheet over them. Put a couple stakes around the pots to keep the sheet from touching the foliage. Mature Meyer Lemons are hardy down to close to 20 degrees.