I'd look at sealing up your door as much as possible as any draft makes it feel loads colder straight away,
these are good for the bottom of the door and something
like this for the top perhaps?
Then if its only for when you're working in there those
infrared space heater type jobs will probably be best, we used to use them when i worked in a big warehouse type place doing PC repair, they don't take too much energy and seem to heat you rather than the space plus stuff doesn't go on fire it it touches them which can be a bonus in a garage.