I would heavily starch the broadcloth before cutting the squares. This will stabilize the fabric when sewing on the bias. Spray starch might help, but I always just go immediately to the heaviest starching solution if I anticipate any distortion using a fabric. My method is to mix a 1:1 solution of Sta-Flo liquid laundry starch and water, "paint" this onto the yardage using a large wall painting brush, allow a couple of minutes to make sure the fibers have absorbed the max amount of starch, throw in dryer, then iron with steam. This works really well on flannel too when you need accurate piecing.