Setup Https with Baikal and Nextcloud

Hello all

I am Running Dietpi as a Baical and nextcloud server on my home Lan

I want to enable Https for all webservices but i dont need a Letsencrypt certificate yet as i do not intend to make it reachable from the internet.
Sadly all toutorials i found only tell you how to configure LetsEncrypt wich doesnt serve my pourpose.

The server is externally reachable through a VPN Gateway running on another pi and i want this to be the only way anyone can reach it.
I still want to do https thought just in case someone snoops on the network portion not covered by the vpn.
Any one knows how?

Oh btw the webserver is lighttpd

thank you forany advice