Usually mountain bike fenders, particularly on suspended forks, are connected to the steering tube of the frame. They're one or two plastic pieces, more tough at the connection side, to resist at a good flexible force.
In this instructable you'll see how much easy is to build your own bike fender with reused plastic stuff. This is not only fun and cheap, but it also will convert at once a garbage into a plastic gadget ready to use, and this is the best way to recycle.
Step 1: The Garbage
You can have a different liquid soap container, but usually they have almost the same shape. Maybe you'll be very lucky and your bottle will be better than mine! Plastic on these bottles is more thick on the bottom part, and this is very handy for us, because it means the fender will be though in the right side.
Cut the rounded part with the scissors and refine the border with a cutter, beware to not cut yourself! You'll obtain a boat shape, leave some plastic on the corners of the back sides, so that they will help to reinforce the connection with the frame.
Step 2: The Hole
Now you simply have to drill an hole one inch distant from the extremity. As you notice looking the first image, that part of the container is not oriented as the rest of the piece, this makes your fender oblique, and lets you choose to mount it on the front or the back side of the fork. If you have a different orientation of the bottle walls, you can easily change them a bit over the flames of your kitchen burners.
Step 3: The Connection
It's time to choose the best way to attach the fender to the frame. I tested two different but equally functional solutions.
The first solution, probably the simpler to find, is to use a spider nut of the type you need to insert into the fork pipe to tighten the handlebar on it. This time simply insert it from the bottom in the fork tube. This solution could be anyway unsuitable when you'll need to remove two spider nuts from the tube, one from the upper side and one from the bottom side, both oriented reversed referring to the extraction direction.
Step 4: An Alternative
So the second way could be use a rubber holed elastomer of the right diameter. I already had this one, and it's of the exact diameter, but you probably have to acquire it from an old fork elastomer, or a wine bottle plastic stopper, or a cork. Maybe also a rubber sheet rolled around the bolt will work good.
At the extremities of the elastomer the two washers will squeeze the rubber material so that it will enlarge in the central diameter locking the fender in place.
Step 5: Ready to Ride!
Although I show first the fender mounted in front of the fork the best way to employ it is to attach it on the back side. Indeed this solution avoids that mud and water splatter on your face.
When you mount the front half of the fender you have to check there are no interference between it and the fork arch, this depends on fork excursion, and you have to push down the fork at his maximum compression.
Obviously you can whenever add a second identical half-fender on the other side, mounted over the first one with overlapped holes, so to have a complete and cool protection against spurts.