The correct fastener is a POS sheet metal nut, that only holds onto one thread and will strip out if you put more than 2 ft lbs of torque on it. If you need them, I'll send you some for free. send me a PM with your address. I have lots of them from all the old fillers I've removed.
IMO, Even though the factory nut is a tin foil POS that will strip out if over tightened, I feel that it is still the best choice. As it tightens, it forms to the surface and gives an even clamping force. If your installing one of my rear fillers, you'll end up with four extra nuts. Just use the best ones to re install. If you need some, PM me your address. I've got some originals that still look like new. I can drop them in a envelope today.