It looks like the u shaped piece is part of the background behind the other pieces? I think I would cut a larger piece of the background colour fabric. Cut and prepare the freezer paper pieces for the pieces that go on top and hand stitch them to the background. Cut a slit in the background fabric and remove the freezer paper. You can now do a freezer paper template for the whole piece and appliqué it to the larger piece? When you have several appliqué pieces stacked on top of each other, you sometimes have to " window" the first pieces so you don't have a large build up of layers.