I would get a 3" wide or so piece of pine or poplar from the home center along with some quarter round or shoe molding to use on the front edge. Glue the trim to the board with carpenters glue while either clamping it or nailing with small finish nails if you don't have enough clamps. Then thoroughly finish the whole pice with polyurethane. The poly is mostly what will keep it from warping along with a good wall attachment method.