I’ve had this starred for a while, but I think I’ll set this up soon. It’s looking pretty good.

Oh god. In the future we’ll need open source resistance-like computer part manufacturing.

It’s not quite self hosted, but Soulseek allows you to share share private directories with buddies. Soulseek might require a port forward.

Other than that, there are the many pasteboard solutions that have been mentioned. They’ll either require a port forward or reverse proxy (nginx etc.) to access outside the network though.