I would stick with straight line stitching too, but do it this way: In the center square, do a continuous straight line following the square shape moving into the center of the square (start on the outer side of the square, and rotate 90 degrees at each corner, gradually working to the center where you stop--like making a square maze--you would have to bury your start and stop threads since they would be in the square, not on the outside border), and on each outside corner, do single straight line echo quilting lines across the long side of the triangle, gradually working to the very corner, starting and stopping on the outside border edges so you don't have any threads to bury. I would make the lines no more than 1" apart, preferably 1/2"-3/4" spacing. Hope that makes sense, hard to describe. Love the block!