Put a small amount of rubber cement around one end of the PVC pipe and then place an end cap over it.
Fill the pipe with sand and then use rubber cement around the edge, placing the end cap over the open end to seal.
Drill two holes, two-inches in from either side of the the back of your riding lawn mower. Make sure they are even to one another.
Take one large zip tie, loop it through one of the holes you have made in the back of the lawn mower and secure it tightly around the PVC piping. Repeat on the other side. Ensure the zip ties are pulled as tight as possible to prevent slipping of the PVC pipe.