evelyne, i have made some things like that. i rolled out a thin slab onto a thin piece of cotton (a former pillowslip) and left it there while manipulating it into the folds i wanted. after it set up a little, i peeled off the fabric. i did not say i easily pulled it off and it didn't work the first time but it works. you could also roll a slab onto thin cotton and set a weight in the center and fold up the edges. playing with fabric and clay is fun.
haven't found the mermaid yet, trina.