You’ll need to register your template for use with the Razor layout manager and then you’ll be able to set a layout for that template. There’s an example of how to do this at the foot of the Appearance > Layouts settings page. Here’a another…
$the_template = locate_template('archive-my_custom_post_type.php');
register_context( 'My Custom Post Type', 'my_cpt', $the_template);
In the above example you would end up with a new context under the “Registered Template Files” heading in Appearance > Layouts labelled “My Custom Post Type”, and to the right of it will be a drop-down menu that will enable you to choose a default layout for that layout from any of the existing layouts.
