Wow, looks like the mylar blanket and light bulb worked pretty well!
I think the worst outdoor temps we've had to deal with here, at least since we got the GH was around -15. That time I had to use a propane heater overnight, but that was before we got the oil-filled heaters. That night was a bit of a challenge! But that was the year I got carried away trying to start things too early--heh, January is NOT a good idea here. (I only use my GH for seed starting in the spring, not for overwintering.)
I put the solar blanket material on the outside of my GH, but I was able to nail it down permanently because my GH is wood framed. I've looked at those Harbor Freight GH's and I'm not sure what would be the best way to attach the material to them. Ken?