Lay a piece of self-adhesive felt face down on a flat surface -- stand the urn on top of it. Trace around the bottom of the urn with a pencil. Remove the urn and cut along the pencil line with craft scissors. Remove the paper from the felt and stick the felt to the bottom of the urn to protect your floors.
Measure the diameter of the top of the urn at the highest point of the rim. A circle of this size will make a top for the urn that will give the pedestal a finished look with no gaps. Use a compass to draw a circle this size on a piece of 1/2-inch thick medium density fiberboard. Cut out the circle with a jigsaw and sand the edges smooth.
Measure the depth of the inside of the urn. Cut a piece of 1-inch diameter dowel, 1 inch shorter than this length.
Use a drill and a 2-inch long, 3/8-inch countersink drill bit to make a hole for the screw that will hold the top of the pedestal onto the dowel that will support it. A countersink drill bit makes a conical hole that ensures the screw will not extend beyond the surface of the wood. Clamp the dowel to your work surface in a vertical orientation. Center the pedestal top on top of it. Drill through the center of the top and into the end of the dowel. Screw a 2-inch long, 3/8-inch screw into the hole.
Mix enough plaster of paris is to fill the urn three-fourths full. Mix it according to the directions on the package. Pour the plaster into the urn. Place the top on the urn with the dowel inserted into the plaster.
Fill the screw hole with spackle. Allow the plaster and spackle to dry overnight. Paint the top of the pedestal black.