Cardboard tube or capped PVC are good ideas. Whatever you choose a wash of the interior with any type of petroleum based oil will help prevent the file from rusting, which will surely happen in a road box. Rust on a file will blunt the cutting teeth, making it useless when you need it. The interior doesn't need to be dripping with oil, a thin film will do. I'd do the threaded cap PVC if it were me, more durable and better to seal out moisture.