You can use any plastic water container for a thermal battery storage device. During transplantation, and the seedling state of growth, extreme temperature fluctuation (+ or - 5 degrees) can seriously delay plant development. Acclimation to the outside environment, for instance with tomato plants, can be aided with full water jugs. Pouring water on the plants and adding moisture to the root zone in the ground is not what I am talking about here. A late friend of a friend, Don Elkins, former dean of Agriculture here at Tennessee Tech University in Cookeville, Tennessee showed me a priceless
bit of information I had never seen before. His tomatoes were the largest I had ever seen, the greenest, and the fastest growers around the local area. Mr. Elkins did not do anything special, besides start his tomatoes in early February, and put water jugs around his newly transplanted seedlings.
You can maintain consistent temperatures with 1-3 jugs of water. Water can hold a tremendous amount of thermal energy before it evaporates, and freezes. To keep temperatures constant, and have some water to pour on the plants during a dry day try this tip. Fill up three, one-gallon jugs of water, or any volume size container and place around your new plants during the spring. The random heat spikes and cold snaps will affect your plants and are undeterminable and unpreventable weather patterns. However, what can save you time and tomatoes is placing water in a container, maybe even paint it black to absorb more solar energy, and at night when temperatures cool, the tomato plant will absorb the extra heat trapped inside the water jug.
Hopefully, it is enough energy to defend the young plant from any extreme temperature fluctuations. After the plants become established, pour out the water on the plants. Using this method the whole season will not hurt anything, but only create a constant ambient protective area for stable temperatures, even during the summer, and fall months. When hand watering plants maybe fill the jugs up at the same time, and then you will have to drag the watering hose out to the garden a lot less of the time.