I do diagonal lines or cross hatching in borders like this. I use wide painters tape, the blue kind. Lay a strip on a diagonal of the end of one of your border, across the middle of the square. Then line your walking foot along the tape. Do not try to sew along the edge of the tape as you will sew into the tape and it is very hard to get pieces of tape out of your stitches. Then move the tape to the other side of your sewing and quilt along that side. Just keep moving and sewing until you go all the way around. If you want crosshatching, then put tape on across going the opposite way.