How about a zigzag type stitch.

Start stitching at a right angle for about 10 stitches, stop, pivot, and stitch about 10 stitches at a left angle, stop, pivot, repeat until you have reached the other side. It's important to come to a complete stop so you will have sharp corners, otherwise they will be rounded.

Experiment with the number of stitches and the stitch length. I like to lengthen my stitch length to a 3 when quilting.