Automatically load a specific sidebar in a page template

Could you please tell me how I can customize a page template in such a
way that it automatically loads a specific sidebar?
For example, that all pages that use the template ‘Contact Page’ would
automatically load a custom sidebar called ‘sidebar_contact’.

If I would understand these steps, I might be able to solve my more
complex problem:

I have installed the plugin Event Organiser. This plugin uses the
template ‘single-event.php’ to display events and I would like to
customize this template in such a way that it automatically loads the
custom sidebar ‘event_sidebar’.
No matter what I try, I can’t get rid of the empty grey sidebar on the right side, and it only loads a custom sidebar underneath as a second sidebar. I have created custom header and footer files, because I believe this is where the sidebars
get loaded, but I just can’t work it out.

Thanks for your help!