Hey Nick,
Thanks for reaching out.
Our plugins themselves do not offer any way to add a secondary thumbnail. So to do this, you would need to:
• Use custom code
• Use another plugin that adds multiple thumbnails
We are unfortunately not able to help with custom code, as elaborated upon here → https://theeventscalendar.com/knowledgebase/what-support-is-provided-for-license-holders/
So if you are not able to write custom code yourself, the only options are to hire someone to write custom code for you or to use a plugin that adds this sort of functionality.
That plugin you linked to looks promising; try it out and tinker around with other plugins if need be. If you have specific issues with those plugins, like they don’t work on Events post types, then contact the support team of those plugins about why their features don’t work on custom post types.
Best of luck with your project!
George