We used to use one of those plastic storage cartons, right side up and tied down to the lacing. We cut out two opposing sides to create a cradle to fit the shape of the mast and tied the mast to the crate. (This was a FREE solution, because I had found some discarded crates behind a store.)

One word of caution: In general, for trailering, it is a good idea to NOT carry your mast with the sail track facing down (especially true with Comptips.)