A few custom modules for NixOS.
Check this folder out somewhere (e.g. /etc/nixos/extra-modules), then include the files in configuration.nix as part of the imports list.
Also, Infinisil's super-useful little helper, from this thread can make this neater if you have many. Include /etc/nixos/extra-modules into the imports list instead, then drop that default.nix into this folder.