I have similar setups that are working well. I have a variety of covers on my raised beds - some are plastic coldframe boxes purchased online, others are plastic tarping over tubular metal frames from a job lot store. On a couple of others I used bent hoops made from cpvc pipe tarped with polyethylene sheeting and held down with bricks all around the base. I have used "row cover" or "remay" from time to time, but clear plastic really heats things up nicely. If your temps get that cold I would not try to prolong a tomato - just my opinion. Face it - when they're over they're over. They won't grow well if you can't keep it really warm for most of every day, and the fruit flavor will suffer. Plus, the pollinating insects will not be present. Have a good time with your hoop house project. Fresh salad in December is mighty impressive!