A Brick In A Box..

(Completely Edited From Origenal Post))
A small, single story, flat roofed high mass building with a dark finish.
This structure would be inside a green house framework with roll up sides/mylar film for the hot months and clear greenhouse plastic for the cold ones.
Water tanks could be built the thermal mass of the building. That mass could be covered with Insulative blanket at night to retain and channel the heat.

Thank you, excellent info in large quantities. I think I will be going down that rabbit hole soon...

Agreed! Thats a wonderful link. Northern hemisphere folks should be aware that winter sun is to the south.