I built a collector to heat a 20 x 18 shop some years ago.
I built the collector on the South facing side, using scrap wood and plastic (it was slated for demolition and so only needed to last a couple years)
I painted the section of the building flat black, then framed it out about 6" and covered it with a couple layers of plastic.
I cut a hole at the bottom and top, opposite ends, in the buildings wall, and installed a fan connected to an adjustable fan switch (temperature controlled).
It worked very well when the sun was out.
It looked not so great.
If I ever move to a place where the neighbors and appearance aren't a concern, I would build another.
A back up furnace is needed...
When I built my next garage, I installed a hundred feet of 4" plastic tile thinking I'd install a fan switch and fan to cool/heat the garage with the air from underground...
It all went wrong when I failed to consider the high water table, and didn't seal the plastic well enough.
The whole system is full of water.
My next thought was to install a small pump and run the water through a radiator, like from a car, and put a fan behind it...
17 Years later, I still have it on my list of things to do.
Rob
Waiting for "someday" will leave you on your deathbed wondering why you didn't just rearrange your priorities and enjoy the time you had, instead of waiting for a "better" time to come along...