Hello christopher_oliver,
Please refer the below code to wrap the block in a container:
<referenceBlock name="page.wrapper">
<container htmlTag="html_tag" htmlClass="your_container_class">
<block class="Ves\Megamenu\Block\Menu" name="catalog.topnav" after="header.container" template="Ves_Megamenu::topmenu.phtml">
<arguments>
<argument name="alias" xsi:type="string">menu-top</argument>
</arguments>
</block>
</container>
</referenceBlock>
You can change html_tag of container with your prefer html tag and your_container_class by the class name which you want to assign to customer.
Hope it will help you.