Mount a tube between centers. Take a small diameter piece of hemp rope and glue it on an actual center. Spread some 5 min epoxy on the tube. Turn the lathe by hand wrapping all the way across the tube to the next center and use a dot of CA to glue the rope to that second center, then let the blank sit and cure for 20 minutes. Once the epoxy cures some, then pour thin CA all over the blank to soak into the rope and stablilize it. Once the rope is stabilized with thin CA, cut the rope off the centers and trim the rope back close to the tubes, then barrel trim it. If you want the pen to be smooth all the way across, then use several coats of CA to build it up. Otherwise, send the blank to someone for casting if you are unable to cast a blank yourself.