2.6 ounces (or less) out of ordinary nylon. This is a simple project for anybody capable of sewing a straight seam.
First, I cut a piece of waterproof nylon fabric, and it can be any type, but the lighter the better. The dimensions are not critical. I cut the piece for 34 inches long and 14 inches wide. Fold that in half so that you have a doubled piece 17 inches long and 14 inches wide. At the fold, we will call that the bottom edge, and at the top a casing must be sewn into each of the two edges. Do not sew the two top edges together. I made my casings very broad at about 1 inch. I made the casings on the "inside," and this is currently inside-out. Still inside-out, I sewed a seam down each side. Normally I use a straight stitch with a zigzag over the top of it. Now turn it from inside-out to outside-out.
Down along the folded bottom edge, you have two lower corners. Take a couple of one-inch squares of grosgrain or similar heavy material for reinforcement, and fit those squares into the corners. Using a grommet tool hole punch, make holes all the way through the nylon and the squares. Then fit two-piece metal grommets into the holes. With the grommet anvil and spreading tool, mash the grommets to fit very tightly in the holes.
Now take a length of parachute cord that will become the shoulder straps. Stick one end through the lower left grommet, then run the rest of it up to the top, through one side of the casing to the right side, out of the casing, 180 degree turn, back into the other casing, back to the left side, out of the casing, and back down to the lower left grommet hole. Run it through the grommet hole in the same direction as the first end of the cord, cut it, and then knot the ends together very tightly. The knot should not slip through the grommet. Repeat this for the right side identically with its own length of parachute cord.
If you did this correctly, you have two (doubled) shoulder straps represented by the cords on the sides. If you pull the cords near the top, they will draw the top opening closed.
If this thing had to carry much weight, you would want some cords broader than parachute cord. However, this thing is probably not going to ever carry more than 5-6 pounds, so it really doesn't matter.
Finished, the daypack is 13-14 inches wide and 16-17 inches top to bottom.
This can do double duty as an ordinary stuff sack, large size.