This will usually protect your plants down into the mid 20's. Be sure to remove any coverings when the warm sun comes back out, if not your plants may roast in your makeshift greenhouse. Then place the fruits on some layers of newspaper in a box or tray in a single layer. Then, place some more newspapers over the top of the layer of tomatoes. This traps in a natural gas called ethylene which is given off by the fruits themselves.
Ethylene also triggers more ripening among other fruits.
Some people place an apple among the tomatoes which also helps to give off this ethylene gas. Check the fruits every other day or so for ripening and remove any that are getting soft or showing indications of rot. The fruits with darker green shoulders are the best at ripening and usually have the best taste. Another alternative is to look in a recipe book that might have some uses for green tomatoes.