How can I keep my paint from freezing in an unheated garage?
Make an insulated, heated enclosure for the paint. Since the paint is already on a wire rack, just insulate the floor and enclose the bottom shelf of the rack. Use a lizard heater and a thermo-cube.

Cut and notch OSB, rigid foam insulation and some scrap pegboard to fit on the floor below the shelf. Use scrap vinyl to make a serpentine channel on the pegboard, then lay the heater in the channel, so:



When you put another piece of pegboard on top of this, you'll have a heated, insulated floor that's safe to drag heavy buckets around on.

Position a shelf above the bottom shelf, cut and notch OSB to fit above that shelf and insulation to fit below it. Secure the insulation from the OSB side with coarse-thread drywall screws.



Cut a back panel from OSB and put two rectangles of insulation on it to fit between the shelves. Secure it to the legs of the wire rack with cable ties.



Do the same for side panels. Put some paint in there to see how good it looks.



Add another side panel and a front door that matches the back panel. Secure everything with cable ties, except on the front door, where a pair of hook latches lets you open the door.



Hook the iguana heater up to the thermo cube and you're done.