I'd have to agree that the mud flaps will be the problem. Maybe rivets on some flat medal to mud flaps, use auto double sided tape then once the mud flaps are stuck on go over the sides and top of the mud flap/fender with silicone adhesive to help hold the mud flaps on which would seal the tape from the elements. The running board not only are held on with bolts, double sided tape, they also use silicone to seal all along the top of here the running boards are attached to the rear fender and body. A good silicone adhesive alone is pretty strong.
Maybe drill a few more holes in the mud flaps and force the adhesive through the holes then attach them to the fenders?
Butch
The tape that I'm looking at is 3M™ Automotive Acrylic Plus Attachment Tape. It's made to withstand all the elements, but still have to check on whether or not it sticks to the rubber. If not, then I'll attach something to the rubber mud flap that it will stick to. There's a pretty interesting video on the adhesive they use and how elastic it is and still keeps the bond. Figure this is a good thing since the mud flaps will be moving around while riding. Had already planned on using caulk along the edges, but the silicone adhesive should be a better idea. I REALLY not want to drill through the fender or have the bolts show, so I'm hopeful this works.