Here's how I put the Super Motard fender on the Honda XL500R. First, I bought it at Cycle Gear. It's madeby Acerbis and it is a generic Super Motard fender, which means I had to drill the mounting holes and modify it to fit the Honda. I couldn't locate a fender for the 1982 XL500 R which would just bolt on without modification.
The stock Honda fender has a groove in it, where the wiring from the instrument pannel is routed to the engine. The generic fender had a groove in it, but it didn't go all the way to the back of the fender, and it wasn't deep enough, so I heated up a round-shaft phillips screwdriver with a torch and rolled it over the fender to melt a deep groove into it. If you're careful, the plastic stays smooth. I also shimmed it up about 1/4" with some thick rubber fuel line to create a little additional space. The wiring fits fine and has a little room to move around like it did with the stock fender.
Front view above. Back view below.