I used a pvc rain gutter and cut it every 6" and bent it to fit around the wheels. My local LBS helped me install it. Not sure it's the most aesthetically pleasing thing, but it certainly works and is quite rugged.

The other option I considered was cutting a 29er finder down the length and adding a strip of plastic or metal to widen it. A friend is working on this and it does look better than the gutter, but it is not a sturdy.

Third option seems to be a place called Big O manufacturing that sells a fatbike fender.