I have done a shadow trapunto with organza but I did it over the whole block so that the edges were enclosed in the seams. Is there any way you could do it so the edges are under something. If you cut a silhouette out of the background fabric and fold under the edges, could you then put the organza over the shape and glue the edges to the back of the shape? It would look like just organza if you matched the shape underneath to the background.