Unfortunately, this is not working for me.
I’ve created a new page in the Child Theme called template_fullwidth.php, and added the code you provided above, but I don’t see that it is making any changes to my site.
Firstly, I don’t see the Sidebar Widget as something that I can choose and further, if I did, I would know what the code was to put in place of ‘your-sidebar-slug’
In the meantime, I’ve just created full width pages with two columns (one 75% one 25%) and put a HTML Widget with twitter feed code in there followed by the Search Widget.
Not the easiest way to do this - especially if I want to change or add something.
The other thing is, my site is being developed locally, so I may have to wait to make these types of changes until the site goes live or is at least in development stage and hosted on the Internet.