Their dual purpose is to keep the rabbits clean of any "spray" painting attempted by a neighboring buck and also as a spread of disease deterrent.
We buy rolls of plastic (like used to insulate house foundations), 24 inches wide, just the same size as our cages are deep so all we had to cut was it's length. Staple to 24 inch boards and place a cup hook on each end to secure the plastic to a cage top.
When dirty just spray clean with a garden hose and hang in sun for a few minutes. We also use plastic on the tables the stackers sit on to keep the wood clean, pee-free and dry.