You can use a LARGE garbage bag for "light freezes", esp. if you secure
the bag around the plant, making sure the plant does NOT contact the
bag. You want to avoid thermal conduction in this case. If the plants
are large, you can try packing loosely crumpled newspaper around the
plant (to keep the bag from touching). Also a couple gallons of
boiling hot water (in closed containters!) under the bag will help.

Without some way to store heat, you won't be storing much daytime heat
for use at night unless you cover the plant with a cloche of some
sort..

Good luck!