Piece of wood to use as a shelf (New Marston OX3)

I need a piece of wood to use as a shelf in an understairs cupboard. It's to support a mini freezer so doesn't need to be smooth or beautiful. It will rest on existing battens so needs to be 66/67cm wide, but I can cut it down to fit if too large. Depth is less critical but ideally at least 45cm to support the full depth of the freezer. Thickness unimportant (the freezer isn't very heavy).