You can get a free cert from letsencrypt using their dns challenge. No need to expose to the internet. Add a DNS record that points to the address of your LAN and it’ll make things even easier for your guests.
For the permanent installation case, it's typically easier to use mDNS domains since they're shorter. 'mediapc.local' is easier for guests to type than 'mediapc.local.mort.coffee' or whatever I'd end up with.
What would be a good solution is self-signed certificates, but that too is a non-option until all browser vendors downgrade the warning from a "Someone is trying to hack you!" style scare screen to a more informative "this is a self signed certificate, do you trust it?" style warning screen.