First thing I would do is put some plastic on the ceiling. Second put some insulation up there. Third drywall the ceiling, even if you don't mud and tape it. Then use a heat source. The heat will just go straight up and not do much good unless you close in the space. You could probably do all that for easily under $300. Then you could probably keep a manageable 50 deg with a small heater. Trust me, it's money well spent.
The insulation would also help in the summer with keeping the temp down in the garage. With the attic space exposed to the whole garage you effectively have an oven in the summer. The attic space can reach 120-140 degrees. You do not want that infiltrating your work space.
For a heater, that electric type from Northern Tool puts out some reasonable heat. Or there are those oil filled radiator type that work decent, but slow recovery etc. I also have a small electric that's about 2' wide and has fully visible heating element. They are about $30 at Walmart etc. Just have to remove the overheat sensor in the bottom. They put out more heat than the little "old milkhouse" style heaters.
At a bare minimum put plastic up on the ceiling. Then the heat will at least have a chance at staying where your working.
Those little propane heaters work decent for what they are.