I saw one done in a very interesting way. what she did was to put two layers of fabric together, the top and plain muslin. She stitched around the borders of the trapunto with water soluble thread. cut the small opening on the back and stuffed the filler into it. Closed the openings by hand. Layer the backing, batting and the top, now with the trapunto already stuffed. Quilted around the trapunto, freehand and very tight. Spray the quilt with some water to get rid of the water soluble thread and the trapunto now looks very fluffy. The new water soluble threads can be used on the top and bobbin on the machine.
Maria