The reference to "automotive gasket" is not correct. In automotive parts stores there are "O" rings available. You can buy a Cokin adapter ring which allows the attachment of their system to a standard threaded lens.

This adapter ring can be pressed in place on the proper diameter "O" ring which fits firmly onto the lens barrel. There are many sizes of adapters and "O" rings available. "O" rings are very cheap and will not mar the lens barrel. They come in different inside diameters and thicknesses and allow for a good fit.