John just so you know, I'm not an expert on any of the many topics I post about.
I just more or less approach problems by trial and error. You can always try it and if it doesn't work, then you'll know.
Considering your local weather and the abundance of wood, I would personally double mass the oven as you had propose, double insulate, and double enclose to protect against the extreme weather. That should allow you to hold the heat in longer for baking the next day. I'm not sure if different types of refractory material hold heat in better or not. If you decide to cover a precasted oven with refractory cement, you may consider mixing in some vermiculite into the mix. The % to use I have no idea, but someone on the forno bravo forum would have that answer.
As far as a potential form shape, I did see some rather large plastic igloo shape dog houses at Lowes/HD. I'm not saying it would work, but I did wonder if it would work as a potential mold for a DIY home oven.
Going off of Tom's idea, you can also add cut bricks that you would mortar to the outside of a precasted oven to build up the mass if you weren't confident in mixing refractory cement with vermiculite.
You may not be so inclined to use the oven if you are knee deep in snow or the elements outside so you may consider building it inside an enclosed patio of sorts.
Good luck and do keep us posted.