More feathers on satin

My first attempt at quilting echo feathers in contrasting thread - I need to work on the curve of the spine, looks too clumsy and bumpy. I had marked them roughly with chalk but it wiped off and I couldn't be bothered to redo it - deserves me right!
I filled the background with Leah Day's tree roots, practising my travel stitching, but to be save in black on black. I might make a second piece instead of a plain back for the cushion cover, it's not perfect but not horrible enough to not use it! On the side I tried out a few different fillers before I settled on the tree roots.
Any suggestions how to improve that spine and what other fillers I could use for the second piece? Rhiannon did pebbles in a recent feather piece, but after that placemat recently I needed to give that a rest lol.

PS: In reality the piece is actually straight, just looks funny in the shot sorry!