Add widget area to genesis_entry_footer

functions.php

<?php

genesis_register_sidebar( array(
    'id'          => 'genesis-box',
    'name'        => __( 'Genesis Box', 'themename' ),
    'description' => __( 'This is a widget area that can be placed after the post', 'themename' ),
) );

add_action( 'genesis_entry_footer', 'sp_genesis_box', 9 );
/**
 * Callback for Genesis Framework 'genesis_entry_footer' action.
 * 
 * @return void
 */
function sp_genesis_box() {

    if ( is_single( ) )
        genesis_widget_area( 'genesis-box', array(
            'before' => '<div class=”genesis-box widget-area”>',
            'after'  => '</div>',
        ) );

}