[JLMounce]

I would lay odds there are holes there already, but they just aren't lined up with the fenders. That said, that area is pretty robust and it's hard to move. If the fenders are fitting well already, I'd consider drilling holes. There's nothing behind there that you're going to hit with a drill bit. It's just empty space. I don't remember exactly how, but you can get your arm and hand in there to get a wrench on the bolt head you'll need to use.

Or you can try using a clip nut as well.