Custom headers on cpsrvd

My Cpanel/whm powered server falls under a larger government policy which has updated its security policies and its now performing weekly audits / scans. In order to be compliant, I need to add a custom response header to all responses returned by cpsrvd (specifically: Content-Security-Policy: frame-ancestors, although it's possible others will be added in the future) I am not finding where to add a custom header; googling for the subject mostly leads to a binary option to add a preset X-Frame-Options in WHM tweak settings. Does anyone know how do I add/modify a custom header to cpsrvd pages? There's a somewhat related post here: