They don't work any better than duct tape, but if you're looking for the least expensive way to join the Sackville Clan while simultaneously classing up your bike, you can stop your search here. A few different colors to choose from, each with a reflective strip of 3M Scotchlite. Two grommets. Ream or drill a hole, attach with zip-ties (not included) or twine or wire or you name it. They won't last forever, but they're made of tough canvas and leather, and all they do is sail thru the air and block water, so we'd expect a good 20 years out of them. Should be called "waterflaps."
Mounting holes are 3/4" (2cm) apart.