I keep a heater on frost-watch in my unheated sunroom. It turns on if the temp dips to freezing. Perhaps that would work in your garage? (I bought it at Target.)

I have a caladium to overwinter - but I'm just going to keep it as a house plant. I suspect you can keep the bulbs at less than 60 degrees, just not the plant (leaves and stems). I've heard that the bulbs do turn to goo if you let them freeze.