Soak the bottom of a terra-cotta pot in water for one hour to soften the clay for drilling. It might be necessary to enlarge the drainage hole in the pot to accommodate the pole.
Apply masking tape around the upper outside edge of the pot. Spray the inside of the pot two or three times with extra-thick, aerosol acrylic sealer to make the pot waterproof. Let the sealer dry between applications.
Enlarge the hole with a conical rasp bit, if necessary.
Cut the PVC pipe to the length you want. Insert it into the hole of the terra-cotta pot. Push the pipe through far enough that you can place the cap on the end. Apply PVC glue to the inside of the cap, and place it on the end of the pole.
Add a bead of marine-grade sealant around the circumference of the pole where it meets the hole of the pot. Allow sealant to dry before you put anything in the pot.