I havn't done it yet, but I've used plastic to line a dump truck bed to make the dirt come out and it works great. The plastic is HDPE (High Density Polyethylene). It comes in various thicknesses and sizes. It is usually roll stock 4 to 10 feet wide and whatever length you want. Available at many plastic supply houses. A thickness of 1_8 inch should be fine for a header. You might go to Yesterdays Tractors board - we have a discussion going there as well.