If you have enough extension cords, go to the pet store and grab the biggest submersible aquarium heater you can find. They will open a surprising amount of water, especially if placed near a portion of water sheltered from wind. It doesn't need to be deep down, the convection around the heater will provide circulation.
It wont clear a whole pond that size, but a guess would say a 250W heater will knock a 20x20 or more hole in the ice unless it is below 20F with wind.
I cleared off an entire 10x6 garden pond in the middle of January with a 100W heater, and it was very cold at the time. I tried to save the frogs and failed when the power went out, and it froze right back up.
