If you're putting these behind a reverse proxy (nginx, etc.) you can just setup client certificate authentication by using your own locally generated CA or by using something like Vault for UI-based certificate generation. When you visit this site with a certificate installed on your device, it will authenticate successfully, and for those who do not have a correct certificate installed, a "No certificate presented" error will be shown.

It's fairly easy to setup and there are multiple guides available for it. Here's one: https://fardog.io/blog/2017/12/30/client-side-certificate-au...