I've started keeping all of my large lenses in dry sacks with a packet of dessicant thrown in. I line the bags with half an inch of polyurethane closed cell foam with a double layer on the bottom. In addition to the obvious advantage of storing the lenses in a dry space, dry sacks are very common airline carry-on items in Alaska. I've never had to argue with flight crew trying to take one from me and stick it in the hold. Which, believe it or not, I did a few times with my 500mm in a camera bag. The 300 2.8, 200-500, or 500 Pf will fit in most 20 liter bags lined as described or with body attached in a 25 liter bag. The 500 f4 without the hood takes a 25 liter(same diameter but longer than 20L).