YOu can grease them well, both inside and out, and leave them in a cool, dry place- like the rafters in a dry basement. Or you could use a PVC pipe, but get some of those desiccant packets sold at hobby stores, and sometimes in general merchandise stores. You can also reuse any packet that comes in anything you use or buy- often found in boxes for electronic components- cell phones, phones, DVDs, etc. Put them on a cookie sheet and dry them out in the oven at low heat for an hour. They are then just like new. I have several barrels for one gun, but they are oiled, and left in a closet in a very dry room.
You are fighting two problems that work together, sometimes to ruin metal. Moisture, and dust. Put the barrel somewhere that is not going to get a lot of dust on it. Then make sure that space is also kept very dry.
Your idea to use a PVC pipe with end caps is very good, as it serves both purposes. If I had more than one extra barrel for a gun(which I do), and was concerned about them, I would consider buying a large- 5 or 6 inch diameter PVC tube and end caps, and then insert dividers made of light wood, to keep the barrels from banging together, and put all the barrels in one tube.
