Do you have a serpentine stitch on your machine? Or you could
do some organic waves like SuzSLO mentioned. I would go wavy
vertically and horizontally. It would match the modern look and
you won't have to worry about the ditch or uneven seams.
Take a piece of clear vinyl and a black sharpie and draw those wavy lines and see how you like it. You can erase them with
rubbing alcohol and start all over. Just make sure to use a large sheet of vinyl so you don't accidentally draw on your quilt top.
I also tape blue masking tape all around so I don't draw over the
tape and I know which side is up (for next time).
Keep it rolled on a poster tube for future use.