Hi,
If you main garage door faces south, you could use this scheme:
Solar Workshop or Studio
It really works well -- provides both great heat and light. Its my all time favorite solar project.
You can work the thermal mass some by placing some water filled barrels in the opening where they will get some sun exposure. I logged some temperatures with a couple barrels, and could see difference -- I think the plot is in the article. It might take more like 4 to 6 to get the kind of temperature swing you want.
Where is the southern exposure that you have to work with relative to the garage you want to heat?
Gary