Believe it or not but SITD seems to be harder than folks think it would be. I can't stitch a straight line to save me life even when using a foot with a flange to keep me in the right directions. Something I did find easier and even fun to do is crosshatching, going from corner to corner. I realize your individual blocks are more rectangle but you could put 2 crosshatched patterns in those and it won't take away from your shell. You might try straight lines going down on the fabric surrounding your shell too.
Nice work on your quilt.