Nice! It might be a good idea to set it up where it's going to go and monitor the Max/Min temp inside for a couple of days. I believe what in under the cold frame will affect the performance/temp. Gardeners used to make "hot beds" by burying fresh manure, if you bury the bottom edge in soil that would seal it more, if you set it on concrete or brick surface, it would sequester heat (is that called heat sink?) Hmm what else... if you set it on a slope facing south, etc. etc.
I put together pvc pipe hoops covered with 6 mil construction-grade plastic sheeting. I'm using it to harden off my cold weather transplants. Due to chicken wire fence around the bed, I can't completely bring the sheeting down to the ground on two sides. But for the last two days, the temp inside have soared to 80's (I do have a little thermometer inside it that I can look at). I've had to bring the transplants still in their pots OUT of the mini-polytunnel. (In fact, yesterday, I brought my older tomato transplants out to enjoy the heat in the mini-polytunnel for a few hours.) But the last two nights, the outside temp have plummetted to 34Ã‚ÂºF and 32Ã‚ÂºF
I don't know what the temp inside the structure was, but the cold weather transplants came through OK.
Yours is much more solidly constructed and better insulated, so keep that in mind too. I hope other folks with more experience respond. I'll be tuned in.