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// Remove after entry widget, so you can add to it
remove_action( 'genesis_after_entry', 'genesis_after_entry_widget_area' );

// Add after entry widget to posts and portfolio entries
add_action( 'genesis_after_entry', 'amethyst_after_entry', 9 );
function amethyst_after_entry() {

   if ( ! is_singular( array( 'post', 'portfolio' )) )
        return;

        genesis_widget_area( 'after-entry', array(
            'before' => '<div class="after-entry widget-area">',
            'after'  => '</div>',
        ) );

}