Wow. Not bad!
Had you considered filling the oncovered space with leaves to form an insulation blanket? That may even work better than if you had that side covered with a shower door.
Our Brussel Sprouts that were growing too slowly have survived 0 degree F so far. Though a little frostbit! The sprouts are still only marble size.
But no, I would never have thought of doing this with Spinach!
Thanks!!
I plan on building a few proper cold frames and doing row covers this next season. Last year I was harvesting spinach the begging of April until late June and saved the seed once it bolted. I think I’d be able to harvest from February until June with the right planning and cover.