A doubled piece of 3/4" plywood about the size of the factory Park base plate. Glue and wood screw the two pieces together to make a 1-1/2" thick base and bolt the Park's base to it with the biggest diameter carraige bolts that will fit the holes. Insert the bolts with the heads underneath and counterbore the underside of the wood so the bolt heads are flush. Use washers and nuts to fasten the upright in place. Sand the edges and corners smooth and paint the base with whatever paint and color you happen to have lying around.