We got a DIY one right now. Huge bucket filled with foam at the bottom, smaller bucket covered in chords with heat.. putting that bucket into the big bucket. Works like a charm. Using a double pall by the fence to keep the extra chord in under the fence so that the horses won't accidentally get in touch with it.
Also, a plastic ball in the water works alright and keeps the water in much more movement when they drink.
And don't fill it up with hot water - that just freezes faster.