Set Default Sidebar Layout Programatically

Hi, I'm using a function to set some default customizer options including:

$cust_opt['underline_links'] = 'never';
$cust_opt['back_to_top'] = 'enable';
$cust_opt['dynamic_css_cache'] = false;
$cust_opt['smooth_scroll'] = 'true';

Which works fine, except I cannot seem to add an option to set the default sidebar layout to 'Content (no sidebars)'.

I've tried using $cust_opt['layout_setting'] = 'full-width'; and all other 'common sense' values to replace 'full-width' but cannot get it to work. Can you please help?

Thanks, David...