Hey WnB...some good suggestions, but would agree with BC...a heating pad really won't be of much help. You didn't mention the size of your containers, but a heating pad is best for containers under two inches [my
].
Again, not knowing too much about your set up [i.e. is it contained (cabinet, pannels for walls w/open top, etc.) or out in the open? I'm at a loss to try and offer advice. But, not being one to let you go away empty handed, why not go by your local Big Box One-stop and/or home center and pick up a small ceramic heater with a thermostat. They are not a big purchase item, and might give you the peace of mind and solution you are looking for...
Good luck!
Nelson