Another thing I do when we have some really cool nights in winter is to set clear plastic jugs of water around the plants. The sun warms the water in the daytime, and the water releases the heat slowly at night, keeping the ground and surrounding plants warmer. Portable heat sinks, in effect. The bigger the jugs, the longer they stay warm. We buy drinking water in 2.5gal. containers for hurricane supplies and after we drink the water, they make fabulous heat sinks. I put my used coffee grounds in the water inside them, which keeps mosquitoes from laying eggs in the water, and also makes the water dark to absorb more heat.
You could also water your containers with warm water last thing at night to help keep the plant warm. (don't wet the leaves, though) The ground is The Big Heat Sink as long as the sun is warming it in the daytime and damp soil will hold heat longer than dry soil, too.
Here's one of my little greenhouse shelf units with three jugs on the bottom shelf. They warm up nicely during the day, then when I cover the shelves with the zip up clear plastic, they keep the whole unit nice and toasty at night.
