[Espeefan]

If you don't want to drill holes through the fenders, you'll have to epoxy the brackets or mounts you make, to the inside of the fender. The brackets could be done a few different ways. I'd consider a piece of aluminum round bar, drilled and tapped in the center of one end, for a single screw, to bolt to the truck's chassis frame rail. If you made two of these for each fender, they could be epoxied to the inside of the fender, and I think it would be a pretty solid mount. Make the mount out of some smaller diameter bar stock, but large enough for a 2 or 3mm screw to be threaded into it.

Or, take some narrow and light aluminum flat bar, or even brass, and simply bend a short 90 degree end on it, so that bent part can bolt to the chassis frame rails, and epoxy the other longer flat to the fender. I'd use two per fender, for strength, and to keep it from twisting.