Thanks so much for the thoughtful reply. Lots of good ideas to consider.
I've actually ordered a custom cover (tarp) made of woven UV-resistant greenhouse plastic (similar to "Bob's super strong" but from a less expensive supplier).

Our garden spot gets full sun, but we're in a canyon setting so sunset comes early as far as the plants are concerned. It will get ragingly hot in there on the warm days - I plan to build end flaps that can be opened and then once we're past the heavy frost danger, roll up one or both sides to allow some ventilation that way. At some point in the season, we'll roll it up completely or even replace with a sun blocking tarp and it will become summer shade. Currently it is a freakish 69 but we'll get some more snow and freezing here (our safe last-frost date is May 8th)

As for heating on the coldest nights... not sure. I like your suggestion of little hoops inside the big hoop. That could be all that is needed here. Also wondered about water jugs next to each plant, to get hot during the day and radiate some back at night. I've read about doing this with milk jugs painted black.

Wind is a big concern. I imagine I'll be fighting that for a while no matter what. I love the pipes in the soil idea... maybe next year : )