I did think not for a studio but for a workspace. Three things are needed, insulate, stop drafts, add heat. For insulation I used regular fiberglass and covered with plywood. For air infiltration, the garage door was the hardest to seal but adding rubber seals to the door made a pretty decent insulator. For heat, stay away from the gas/propane ones and use those oil-filled electric heaters. You don't want to be breathing fumes in an enclosed space though I guess for a studio, the lights might be enough by themselves.
For a space "part of garage," in a place of moderate climate, I'd also recommend oil-filled electric heaters, even two or three.
If we were talking about a permanent studio space in a place that spends a few weeks below freezing, I'd recommend a "real" HVAC system.