hey man a friend of mine had the same issue, he used these insulating bricks for a floor of the cab. i think they are the kind that you line your woodstove with, anyways the heat from the lights during the day warmed the bricks up and they radiated heat all night.
also plants are pretty resilient and as long as the roots dont freeze they are pretty hardy with cooler tempratures. last summer my plants survived through 3-4 heavy hitting frosts. ended up having to chop early due to bud rot but the frosts didnt kill them.
also a reptile heating mat in between them bricks works well too and they are probally only 100w you dont need to maintain temps in the 60-70's just keep them above freezing.
another thing you can do is run the lights at night when the coolest temps are about, then lights off during the day while its slightly warmer.