Wow… this is something with which I actually have some experience, so here goes…
As saltwater says, you need silica gel packs before beginning this process.
As for me, I use Ziplock bags, the ones for freezer-use in quart and gallon sizes (mostly quart bags for the small primes, a K-7 will fit in a gallon bag). I place the lens into a Ziplock bag, drop in a silica gel pack (sometimes two packs), partially seal the bag, squeeze out as much air as possible, then complete the seal of the bag.
But, wait, there’s more…
Since having a bunch of bags sitting on a shelf, or two, is a little inconvenient, I use Lowepro camera equipment backpacks into which I place the bagged lenses/body. After filling the backpack(s), just zip ‘em closed and the equipment is easily moved/transported in cushioned comfort.
But wait, as an extra, added bonus…
I also have a Lowepro, large, really tough-hard plastic, airtight container which is large enough to hold the Lowepro Pro Trekker AW backpack. I just left a message at Lowepro to see if they still sell this unit. I also have a Doskocil really tough-hard plastic, airtight container which, although smaller than the Lowepro unit, will still hold a small camera equipment backpack.
Just got a call from Lowepro: This really tough-hard plastic, airtight container was made by Pelican and the included inserts were made by Lowepro. This unit is no longer available. You can, however, purchase a Pelican case large enough to hold a camera equipment backpack.
For those of you who believe I may have gone overboard with using the really tough-hard plastic, airtight containers: At that time, I was a very active river rafter and “overboard” could be a really big problem for camera equipment. Those soft, comparatively small, rubberized, watertight pouches just didn’t help if they fell into the water and then hit rocks.
I was also an avid 4-wheeler; and crossing streams/rivers which were wide and slow-moving, making it appear as though a crossing would be really easy, could quickly become very problematic if no one walked across the wide stream/river first. Me, I just parked my Jeep, got out my camera with a zoom-telephoto lens attached, and photographed the action. After the first few vehicles dropped into unseen, deep pools and got really water-soaked, or even fell over on their sides, others would “walk” the stream/river to find a safe crossing.
In the past couple of years, I have wondered about some other alternatives, especially when I read about floods in the news. A product that is now available and which seems to me to be an idea worth considering is the plastic, vacuum-compressible, bags apparently designed to store clothes, blankets, etc in a much smaller space than if the items were not compressed. If you can find one of these bags large enough to hold a backpack containing camera equipment, it might be a quick, easy, and lightweight solution compared to a heavy, hard plastic, container.
One source for these plastic, vacuum-compressible, storage bags is DealExtreme.
Look here for one of the “Compact Space Saver Vacuum Compressed Seal Storage Bag”:
DealExtreme: $4.35 Compact Space Saver Vacuum Compressed Seal Storage Bag (60*80cm)
and here for a manual “Air Pump for Vacuum Storage Bag” (as opposed to using an electric vacuum cleaner):
DealExtreme: $4.20 Air Pump for Vacuum Storage Bag
I also use a vacuum sealer to store, and freeze, BBQ ribs, pulled pork, chicken, etc when rib, pork butts, chicken breasts, etc are on a really good sale. I have thought about using this process to store small lenses I leave at home when I am away on a long trip but that seems like a really extreme idea. If I thought I might be in a flood situation, I just might try this idea.
OK, I’m done. While I don’t know the cost of a “dry box”, I hope you find some of these ideas helpful, even if they are not less expensive than a dry box.
RayGunn