I use black number plate plastic bolts and nuts, looks neat (unlike self tappers that rust) and last the course, served me well for last 25 years
I also add a small rubber m/c mudflap to the bottom of the fender extender, to give more coverage and protection and it works well (again held on with black number plate bolts)
https://www.ebay.co.uk/itm/273771572005?hash=item3fbe0eab25:g:W20AAOSw5KBck3Xd