We now sell rolling guides in water resistant green MDF. They are available in thickness of 3mm*, 6mm, 9mm and 12mm, you get 2 per pack.
To be placed either side of flattened piece of clay, put the rolling pin across all of them and then roll to the level required.
Hot clay tip, Clay is best rolled when it has been left for a while, depending on condition of the clay but an hour will usually suffice. Work from the centre and roll forward and then centre and roll back. If you buy a range of guides, you can start with the thickest of them and work progressively toward the depth required which will help keep the clay even.
*3mm Has been specially made for use with paper clay and clay bodies that can be used very thin, however these are made with a standard MDF and therefore not water resistant. But they are still a very good idea.