Fiddling around with some vintage parts I located years ago. These are a set of NOS Pontiac Trans Am mudflaps.
Lest you think I drilled holes in a brand new paint job, here is the method recommended by a friend in town who has a white, original paint 68 Shelby. He has similar white Ford mudflaps and has them held on when driving with these heavy duty Home Depot mini clamps. He's been running with this set up for a while now and hasn't lost a mudflap yet. I may paint the clamps black so they don't shine so much, but other than that they seem to hold the mudflaps in place quite firmly.