Heh. Just kidding. The idea was to make a bag where the sides went straight up to become the strap…but I totally underestimated how long I needed that strap to be. So…it’s more of a man purse. *sigh*
Cut two matching rectangles as wide as you want your bag to be and twice as long (plus 8 inches) as you want your bag to be tall. (That gives it a 4” overlap for the front flap, plus 2” to go over the top of the bag, and 2” to go under the bottom.) Then cut a piece that is 8” wide by the distance from your boy’s knees, across his chest, around and back to his knee. (Obviously, I took the measurement hip to hip…and that gave me a purse. D’oh!)
Sew your two big rectangles, right sides together, making sure to leave a gap for turning. Clip the corners, turn and press. Set aside.
Take your big strap piece and fold 2” in on either side so that they meet in the middle. Press. Fold in half lengthwise, so that your folded edges meet and right sides are out. Press. Sew along the length, right along the edge where the two folded edges meet.
Now, here comes the part that made my brain turn into licorice. At one end of your rectangle, measure in 4”. Mark that. It’s your “flap. Now, measure in another 2”. Mark that. Now you have the two corners for the “top” of your boxy messenger bag. Now, measure down as far as you wanted your bag to be tall…mark that. Go another 2” and mark that. There’s the bottom of your bag. You should be left with a length that is identical to the back of the bag.
Time to attach your strap! Your strap should have ended up 2” wide, and so should just fit into the “bottom” that you measured out for yourself. Carefully pin your strap into place and sew the short ends to the bottom of the bag, right sides together. Then, sew up the sides to close up the sides of your bag.
(If you want a stiffer bag, use something heavier than the flannel and cotton/poly bed sheet that I used or add interfacing between your layers.)
(PS – I obviously sewed the long sides of the strap before the short side. Do whatever doesn’t make your brain turn into licorice!)