More food for thought... Try a heated propagation frame. Might be more
useful in Ohio.

Interesting, but I'd like to stay away from active heating if I can.
However, I may very well place a dozen or more milk jugs of water in
there to help regulate the temperatures.

In my ideal cold frame setup, I'd have a 200 gallon water reservoir
buried underneath the cold frame. Then on the south side of the
building, about 5 or 6 feet above the cold frame, I'd have a network of
black tubules inside an array facing the sun, and a tiny pump to slowly
move the water through there from the reservoir during the day. That
would build up a lot of heat in the reservoir, which could help slowly
release the heat passively through the only non insulated surface - the
top of the reservoir tank. (located, conveniently, only a few inches
under the bottom of the cold frame)

However, that's probably a bit complex for my first cold frame attempt.