Docs for LXC things.
Download from hydra.nixos.org. "-small" releases don't do system tarballs, download a normal release.
Save the release in Proxmox as "nixos-system-x86_64-linux-${RELEASE}-${BUILD_ID}.tar.xz".
For example, the current tarball is named "nixos-system-x86_64-linux-24.05-269898867.tar.xz", which comes from hydra.nixos.org/build/269898867.
Do a normal container setup, and set the IP temporarily via Proxmox.
Open root SSH permissions.
Also make secrets configuration.
A bare minimum setup, meaning a host with:
Might need two rebuilds.
And done!