What I wanted was https://kennyfraser.com to be the definitive URL of this site, and https://www.kennyfraser.com to redirect there.
The GitHub CNAME file contents are just:
Don’t make this mistake in the DNS provider’s CNAME records:
Using a blank hostname in the CNAME records can mess up the MX routing for email, and possibly other services.
The www Host Name CNAME record points to my username.github.io. This is the recommended custom subdomain for best performance.
Two A Records for the apex (non-www) domain with Github IP addresses are also necessary. GitHub Pages automatically creates a redirect between the www and non-www versions of the domain, depending on the contents of the CNAME file.