This is a quick project to make an awesome waterproof bike fender using a half-gallon milk carton and four plastic zip ties.

Required items:
an empty half-gallon (2 liter) milk carton (either with or without the capped spout)
a good pair of scissors
a permanent marker
hot glue gun or comparable adhesive
4 zip ties (available at your local hardware or electronics store)

When finished, the fender attaches only to the seat post stays of the bike's frame

Step 1: Open the Top

Rinse the carton really well.
Open the entire top along the seam so that the top is completely open.

Step 2: Cut It Up!

Cut with scissors into two parts as shown.

The half with the spout (at right) will become the front half of the fender. This is the part that will wrap around and attach to your bike frame.

The other half will become the back of the fender.

Step 3: Back Half of the Fender

This is the back half of the fender.
Cut a notch out of the bottom of the carton to allow clearance for the tire. This might not be necessary for all bikes but it doesn't hurt.

Step 4: Front Half of the Fender

Cut both corners so that the bottom is free from the sides.

Step 5: Attach the Two Halves

Withe two pieces laying top down, set the back half on top of the front half so they overlap by about 4 inches (100mm).

Mark the location and heat up the glue gun.

Apply glue to one mating surface of the carton and press the other half on top.

Repeat for each of the sides.

Step 6: Cut Notches to Fit Frame

Notch the top of the spout as shown.

These should be about 1/2 inch (12mm) wide and 3/4" (18mm) long.

Step 7: Pre-roll the Side Flaps

It helps make installation easier to pre roll the flaps that wrap around the frame.

I used a sharpie to help shape the flaps.

Step 8: Put It on Your Bike

Wrap the pre-rolled sides of the fender tightly around the frame so that it wraps all the way around.

Holding the fender with one hand, make slits 1 inch (25mm) below the top and 1 inch (25mm) above the bottom for the zip ties.