by the second winter (below) I set up a simple frame, added a hinged hatch to be able to keep an eye on the fish, put plastic over it. The plastic is held down with wood battens, screwed down for easy removal in spring. 
I run the pump in a 5 gallon bucket filter filled with some netting and hung from the side of the pond.  The force of the water from the pump keeps the water open all winter. The pond is 4 feet deep, only 1.5 feet is above ground. 

It is painfully bleak during winter. Sigh. See below for the pond in summer.