I searched for a haversack pattern but could not find one, so I have drawn this one on a piece of A4 paper. The straps can be leather or fabric. On leather straps you could also add a buckle for adjustment. For straps I usually get old waist belts from an op-shop, you can stitch two together for extra length. Where you stitch the straps on you may wish to reenforce the inside of the bag with a square of extra material or leather.
My haversack above was made from an old canvas riding coat. The flap has leather sewn around the edge to stop the fabric freying, and it has a three button closure. These buttons are pewter.
The strap is made of leather but it has no adjustment buckle.