simple cheap fix is to change out to a 1/4-20 stair bolt from the local hardware store. make you use a good epoxy or jb weld to hold them into the extension. That's how I have fix all of mine.
go to the hardware store and ask them for them, they will know what you you are asking for. They have a screw thread on one end to thread in to your fender extension and a 1/4-20 bolt on the other to bolt to the fender.
Just to be safe take the fender extension with you and make sure it screws in. Use JB weld or epoxy to anchor them tight in the extension.