A Sea of Green in a 2 x 2 area Shouldn't need more than 16 plants.
That aside, when temps are very low, I put my plants on a table covered with a plastic sheet, to the ground/floor.
A small space heater is placed under the table, and I set the thermostat to keep soil temperatures comfortable. For mature plants, the 40's are adequate. Young plants need temps over 50 degrees.
Clones and seeds need a minimum of 70 degrees to thrive.(Water can be an issue at 70 degrees, so be vigilant.)
Remember, these are soil temperatures.