A haversack is nothing more than a canvas bag with a shoulder strap. Kinda like a canvas hunting pouch, but larger.
Mine is 20 inches long X 14 inches deep. This haversack will carry all of my food and sundry items necessary for a several days hunt, or trek.
And it allows me to carry a matchcoat under the flap.
The body of the bag is best made of one piece of canvas folded in about thirds to form the body and the flap. Narrow gussets can be sewn into the ends, to make a lttle more room for your gear.
If you decide to make gussets, the bag is easier to use if the gusset is tapered, narrower at the top. Two and a half inch wide gusset, at the bottom, tapering to an inch and a half, or less at the top makes managing the bag much easier than a with a single wide gusset all the way up.
You will lose a half inch in seam allowance, but that's ok.
I prefer the flap to reach within an inch or two from the bottom of the bag to protect the matchcoat.
The bottom of the flap is cut straight, and secured by three 1/2 inch buckles and 1/2 inch wide X four or five inch long straps. However buttons work well too. Later haversacks used three buttons to close the flap.
Use the heaviest canvas you can find for the bag, and I prefer a leather strap with a buckle for adjusting the length for use in all types of weather in all seasons.
A haversack can be made any size you want, with or without a gusset, and with a simple canvas strap, if that's all you need.
J.D.