Personally I'd be a bit nervous about using knobby tires with rigid metal fenders, but that's just me.
That being said, I use 650b Berthoud stainless fenders on my 26" wheeled commuter bike and they fit the contour of the wheels just fine with 26x1.5 Paselas.
The chainstay bridge is very narrow and farther from the tire than is ideal, so here I used an aluminum spacer with two leather washers:
At the seatstay junction I used a rubber bumper. Probably could have done it the same way as the chainstay junction or vice-versa, but I didn't have another aluminum spacer of the same dimensions or another rubber bumper like this one:
At the fork crown the gap was enormous because I foolishly chose a Surly 1x1 fork, when I should have used an LHT fork. So I made this spacer from a short piece of 1/2" PVC pipe. The draw bolt is extended with a double-ended nut secured with Locktite, and the fender is attached from the other side with a regular bolt. Very difficult to line up the pieces but it does provide a little extra clearance for the tire. In this photo you can also see that I mounted the little bolts for the fender stays backward and capped them with tiny stainless steel acorn nuts. This also adds a wee bit of tire clearance but I mainly just liked the way it looked. However, mounting all the bolts this way means the inside of the fender is very smooth compared to having bolts and nuts protruding. There's not much in there for debris to hang up on.
