it is gorgeous!!!
?? make the piano keys loop all the way around, front to back?, so 5" would be 10", 4" would be 8". Maybe even 6". Batted, with corners in grey, mitered. I'd foundation the keys with an old sheet for strength.
I self bound to the front 8" on a queensized quilt once, double batted the whole quilt (before they made double bat) the edges are poofy & fun. I'm pretty sure I made it envelope style, wrong side out finished & turned it in & slip stitched the opening closed. That won't work on this one if you have quilted the quilt. Then, I tied it.
If you do the piano keys all the way around, I'd attach it to the back, miter the front corners, turn under the front edge & stitch close to the edge, scant 1/8". The bobbin side, it will be smoother if you are stitching as close to the ditch as possible, on the quilt side rather than the border side.
Mitering isn't too scary. Pin around the seam allowance, bring the corner into position, pinning down the point. Bring the left & right sides together at the point. You can leave it open, or slip stich it closed.