I am trying to find any practical information about design and construction of "solar closets".
This is essentially a very flat greenhouse with maximum glazing and thermal mass, but no space for residing. Its designed purely to get hot, and store that heat in thermal mass. It seems to be a solution for intermittently sunny climates where you can't afford a lot of glazing due to heat loss during cloudy days, and you can't afford or don't want a full greenhouse (I don't like the humidity bump from using a greenhouse both for growing and for heating, and so I'd rather spend on a hoop house, and use a solar closet for heating).
I am deconstructing a rotting sunroom in my c. 1982 passive solar house, and have some large windows and thermal mass in a good position, but cannot afford to rebuild a sunroom, but am loathe to give up the solar gain.
I have seen the Instuctibles design before, thought it would be good on my brick house but my wife wouldn't go for it.
I have considered a junked freezer as a green house.
If one skips the plants in favor of a 55 gallon drum all that would be left to add is glazing, aluminum screen and a way to vent into the space to be heated.