Keeping the plants from freezing was easily accomplished but keeping the soil temperature in the pots above 50 degrees to let the tomatoes and peppers continue to ripen and maybe to set more fruit on the cherry tomatoes has provided a bit of a challenge.
A single layer of 6mil plastic was letting the temperature drop a bit too much at night so I added a night cover of a reflective back tarp I got on sale at Harbor Freight over the summer. Using it has let me turn the heater down to low and still maintain temperatures 10-15 degrees higher than ambient over night. It has been a bit of a hassle putting it on at night and taking it off in the morning but I am finding new baby tomatoes so I do not mind so much.
The entire structure has a double layer of cardboard about 20" high around the base right now but will slowly be augmented with 2" thick styrofoam blocks. I will also try adding large cell bubble wrap to the inside of the roof in hopes of limiting the use of the tarp. Bubble wrap could stay in place all day and actually add a bit to the heating.
Thanks to all who helped find the ideal temperature for the greenhouse as the cherry tomato that went in covered with blooms a couple of weeks ago is setting fruit like crazy--saw 8-10 new ones today. The celebrity tomatoes that were already on the plants are thriving and the first one is ripening. I do not expect more fruit to set. Peppers seem to have a few new blooms but again am not expecting anything but growing out those on the plants already.