5mm pan head machine screws, stainless of course, with a nyloc nut as mentioned above. I know the Parts Source stores around here sell 'em... stainless nyloc nuts were 0.11$ each. I've had to do this with a few French bikes, except the holes are bigger so you can 6mm if your racks will accommodate them.. I usually install my fender/rack screws pointing out, just in case I ever break one off I can just unscrew it and put a new one in.