I'm with you, Mj. Water jugs are my heat sink of choice. We buy the 2 1/2 gallon ones from Costco at the beginning of hurricane season for emergency water, then in the fall we drink the water and I save the jugs to snuggle up to my favorite plants on the cold nights.
My little 'greenhouse shelf units' for the orchids have enough room for 3 of those big jugs in the bottom, and they stay warm all night if I put the hot water in just before I go to bed then zip up the plastic covers.
If it really does go below freezing, I might add a blanket over the plastic cover, too.
