Cut the back of your coat apart down the middle.
Cut a strip of fabric the length of the coat back plus 1".
Fold the short edges of the fabric up 1/2".
Fold the strip in half longways. Press.
Lie the fabric strip on the right side of the coat. The raw edges of the strip should meet the raw edge of the coat. Sew in place using a scant 1/4" straight stitch.
Fold the strip over the raw edge of the coat. Sew in place along the folded edge.
Attach the magnet by creating a pocket or just sewing the magnets to the two top sides of the coat.