I may eventually post a tutorial for this, if only to illustrate to the author of Riding Pretty* that DIY is more than wrapping your purse strap around your handle bars and saying, "It's a bike bag!" However, most of the folks who bother to read this can figure out that it's just a bunch of rectangles and a bit of velcro in the right places, right? Easy peasy, especially if you don't insist on hand-sewing it in front of the babble box.
The body is made of a cotton upholstery fabric, the sides and inner lining of the top flap are canvas. The strap is canvas tape I cut off the useless little Bike To Work Day musette bag I scored at the BART station. And there's a resin butterfly stitched to the front flap just 'cause.
You do need to measure the drop from the rear rack to the chain stay to determine where to put the velcro/how long to make the bag. Depending on what sort of bike you ride (and the position it puts you in), you may also need a helper to measure where your leg is farthest back as you pedal so you can account for clearance. You can see in the top photo that the rear pedal (and my heel) will clear the corner of the bag as it hangs.
The velcro looks a bit wimpy, but it's mostly to keep the bag from sliding around. Wrapping the canvas shoulder strap around and under the "rat-trap" part of the bike rack is sufficent to keep it from falling off the bike with weight in it.
*I do not "ride pretty." I just ride, complete with gray roots, hot flashes and belly fat.