I bought a heated pad for Mr. Casper a few years ago, but the outlet on my deck didn't work, so I ran an extension cord from my kitchen. The problem was that the door wouldn't fully close, because it was one of those heavy duty, grounded extension cords.
Problem? Did I say problem? I just lived with the door never quite shutting for 3 years. LOL. I figured I was on the second floor, and any intruder would have to figure out the pet gate first - and no one can figure out that pet gate. Not very energy efficient for me, but it worked.
Not saying that you should do the same, Cheddar, but if you have an outdoor outlet, and the shed's not too far, you might be able to use a heated bowl. And actually, if you want one, let me know - seriously. I bought one for Mr. Casper but never ended up using it. It's been sitting in my closet for 4 years.
Otherwise, some people have suggested putting one bowl inside a larger one, with styrofoam in between, because it insulates well and it would at least take longer for the water to freeze. I think one person used a tire somehow, though I don't remember the specifics. I don't know exactly where the threads are, but you could try a forum search.