I saved styrofoam trays from frozen meat that I sanitized in soap and hot water, then clorox. (Actually I did that for dd. DS form was a round styrofoam plate.)
Then I had the dc plan what they wanted to paint. Fresco painting must be done while wet and dries quickly...so first they sketched and colored on a sheet of paper their design. I let them choose any design they wanted. It did not have to be a copy of an Ancient Greek design like bull leaping, although that certainly would have been interesting.
Then I mixed and poured plaster of paris into the trays. As soon as they started to set, the dc used their water colors to paint on the wet plaster. After it completely dried we popped them out of the trays.