I would use the string and circle method to make a template on cardboard, since you are going such a large scale. (If it were smaller, I'd suggest freezer paper) I would wrap the fabric around the cardboard template and starch the thing til it holds its shape before pinning to the quilt. You will need to clip some places to ease it around the arc.
I almost think spray basting glue would work better at that point, if the blue line in your picture is thick enough.
Good luck with it. It sounds striking.