Add a widget area above the WP Idea Stream archive page. The full child theme is available here: https://cldup.com/yk8pyyY1p1.zip

functions.php

<?php

/**
 * 1st step: Just register a new sidebar.
 */
function twentyseventeen_child_register_sidebar() {
	register_sidebar( array(
		'name'          => __( 'Ideas', '2017-child' ),
		'description'   => __( 'Above the list of ideas', '2017-child' ),
		'id'            => 'ideas',
		'before_widget' => '<div id="%1$s" class="%2$s">',
		'after_widget'  => '</div>',
		'before_title'  => '<h3 class="widget-title">',
		'after_title'   => '</h3>'
	) );
}
add_action( 'widgets_init', 'twentyseventeen_child_register_sidebar' );

ideastream.php

<?php
/**
 * 2nd step :
 * - create a new template in the theme and call it "ideastream.php"
 * - copy the content of the page.php template and paste it in the "ideastream.php" template
 * - Edit the "ideastream.php" template as shown at lines 14 to 18
 */

get_header(); ?>

<div class="wrap">
	<div id="primary" class="content-area">

		<?php if ( is_active_sidebar( 'ideas' ) && wp_idea_stream_is_idea_archive() ) : ?>
			<aside id="ideas-sidebar" class="widget-area" role="complementary">
				<?php dynamic_sidebar( 'ideas' ); ?>
			</aside><!-- #ideas-sidebar -->
		<?php endif ;?>

		<main id="main" class="site-main" role="main">

			<?php
			while ( have_posts() ) : the_post();

				get_template_part( 'template-parts/page/content-front', 'page' );

				// If comments are open or we have at least one comment, load up the comment template.
				if ( comments_open() || get_comments_number() ) :
					comments_template();
				endif;

			endwhile; // End of the loop.
			?>

		</main><!-- #main -->
	</div><!-- #primary -->
</div><!-- .wrap -->

<?php get_footer();