This is what I call a pond sedan! lol Quite simple to make and very effective too! The pond only freezes up to the box and not underneath.
1 wooden vegetable box with slats.
1 bag of rabbit straw laid on the top of the box and fastened in place with some plastic sheeting and either nail on or staple to secure it.
Next stage is to make a form of apron round the lower part of the box, at least 8 inches in depth. Nail this on or staple. This, when lowered in to the pond gives the fish some protection.
Make some handles, the width of your pond, with which to lower the "sedan" in to your pond.