Although this doesn’t answer your question directly, if your internal host has a domain name in externally visible DNS (eg “something.com”, “something.io”, etc), then you could potentially generate a valid Lets Encrypt certificate for it using DNS based authentication.
That’s how it’s commonly done for servers which aren’t reachable from the outside world.
Can give you the exact certbot commands to run for generating the certificate, if that’d be useful. (?)
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