Cut ¼ inch thick pieces lengthwise from a block of dry floral arrangement foam (a thin blade, like the long flexible blades used for polymer clay works well for this). Glue the pieces to the shelf not covered by the porch or foundation using pva or white/craft glue.. Glue the pieces right up to the edge of the foundation. Let the glue dry.
When the glue has dried, trim the foam pieces with the polymer clay knife so that there is a slightly uneven surface, and the foam is cut flush with the shelf edge. The foam will form a base into which you can add landscaping if you wish.