I've been giving some thought to a passive solar heating system, just enough to take the edge off and bump it up to 50 or so in there. I can't tell from your picture whether the rear wall of your garage is south or north facing, but if it's south-facing it could work. It's a simple approach, use the sun to heat up a large flat air space, have vents going from that space into the garage top and bottom. Hot air rises and goes into the garage, cold air is then sucked in through the bottom vents. Add lightly spring-loaded flaps to close off the vents when there's not enough heat on the outside panels to create convection. Here's a picture of one
sample installation: