Try painter's tape
If you haven't started sewing yet, you can add some 2" painter's tape close to the bias edge on wrong side of the fabric. Don't sew on the tape but place it really close to your needle - I usually let my metal foot run on the tape. Just lay tape down on the fabric in one piece - you don't want to re-position it because of the stretch. The tape will help keep your edge from stretching and if you sew with a short stitch it should be fine.