You may get away with just adding a matching piece to the short area. If not you may be able to buy one for your machine or buy a generic one. It is well worth the expense. If you sew with the fabric just at the edge of the foot, and not beyond, maybe even a few threads closer to the needle than the edge of the foot, you should have a quarter inch seam. If you use several layers of masking tape it makes a little lip to work against.
Electrical tape works well too.
After washing your fabric use spray starch while ironing to replace the sizing. This makes it easier to get that accurate seam.