Painters tape is good for that sort of thing. Lay down parallel rows of tape in the width you want the keys to be and then remove every other one. Do about 6 to 8 inches worth at once and then walk the pieces along the border as you add the quilting. Quilt along the edge of the tape. See below
Quilt along edges of the tape then move strip 1 to space 6, strip 2 to space 7 and so on.
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1 2 3 4 5 6 7