You may not need to line it. Figure if you're storing boxes, and the boxes are made of cedar, that does the trick! The cedar serves several purposes;
1. It regulates humidity (believe it or not). It is one of the most porous woods. It readily gobbles up humidity in a moist environment, and just as readily pisses out humidity in a dry environment. So it helps smooth humidity swings.
2. It helps deter insects.
3. It helps age cigars and imparts a pleasant flavor and aroma.
So stack and pack it, and you should be fine ;-)
If push comes to shove, and you absolutely want to line it, I’d hit a lumberyard. Not a Home Depot, but an actual lumber yard. Find a friend with a table saw, and Viola!