Heavy-duty woven 100% linen couche cloth, 75cm wide (30" so just right for 18x30" trays) and by the metre to a length of your requirement, up to 20m, it is designed to support proving dough, especially baguettes or bâtons. Dusted with flour (preferably rye) to make sure that it doesn't stick, couche helps give the dough a shape and wicks a little of the dough's outer moisture away to help develop a skin to help hold the final shape. In general bakers use a flipping board to carefully remove the fully proved dough from the couche before placing it onto a peel for sliding into the oven.
When finished baking for the day, simply give the linen a good shake outside, or you can, if you prefer, just fold it up for next time. If you need to wash the couche linen, then wash in cold water.