Widget that is not site-wide

It depends where you want the widget to appear. You can always create and register a new widget and add it to show up inside your content area (either up top or bottom). Once it shows up in the content area it will automatically fit inside the width of that area.

There should already be some tutorials out there that can show you how to do this. Here’s one – https://wpsites.net/web-design/widget-genesis/ You can do a google search if you need more, but that link pretty much explains how to do it for all Genesis themes.