Sara in Vienna gives us a hint, replete with pattern, on how to make a simple messenger bag. Her results look great. It’s time to look for a used sewing machine I think.
You only have 4 seams: first you sew sides to bottom, then sides to back. You can make it any size you want. You can line it or not, use a closure or not, make the front as long as the back/flap or not, rounded corners on the front or not. There are many different possibilities for the strapes and where to attach them to the bag.