I seem to have the most difficulty with square shapes. For those I use a more drastic way of keeping the piece from warping. Using sock orphans in all sizes is a good way to rid your laundry room and be frugal at the same time. I take the socks and stuff a plastic grocery bag inside of them. Then I fill the bag with sand and tie a knot in the top. I drape a piece of tissue paper over the work before laying down the socks. You are seeing a women's ankle length in use on a small plate. Several men's tube socks work great when draped long ways on a platter. The only caution I would put out is that if you get too much weight on a piece, it can crack from being unable to move while shrinking.