We use something called Mill Wire. It is a screen like material salvaged from paper mill belts. I would recommend making a bed of sand. Styrofoam is OK although I have had mice decide it is a nice place to make a home.

Plywood is easier than finding millwire...anything flat, off the ground, preferable something that wont be constantly wet or eaten up through the bottom.

I'm using a flat piece of slate only because it was handy but it seems to be working well.