BKT probably meant to say you can have them in 1" INCREASES, in sizes to well over 18" in diameter at the hardware store.
I agree, but that is an EXPENSIVE way to do things.
And, the amount of waterproofing you will need to do will depend on what you are trying to preserve and where you are trying to preserve it...
The Desert is a dry climate, but it's hard to dig through the concrete hard Salt/Alkaloid crust that is just under the surface, so be prepared for a rock hard layer there!
The good news is you don't have to ever worry about standing water in the hole, so turning a water resistant container up side down so the water can't run in and stand when you bury it will probably do the trick.
In ground that is more wet and standing water is a real threat, remember to use a WATER TIGHT container that will maintain a seal for a long period of time.
PVC pipe is good here, since you can 'Pipe Dope' a threaded end cap, or just seal both ends up with caps and glue, but that means cutting open the container when you need something out of it...
Again, seal it up good, and if you have an 'Access' point, bury it facing DOWN so if the seal fails, the container doesn't fill with water and stand there!
If you plan to store underground in saturated ground, you MUST make sure you container won't float!
We had some flooding this spring that brought things to the surface that were never meant to see the light of day again! (Coffins for one!)
Why do you think the graves are ABOVE GROUND in swamps!
Some tips for storage underground...
If you are storing firearms or other metal items,
Make sure you use a second container, storage bag, smaller container, ect, and make sure your metal items are protected from condensation!
Use a storage paper or heavy grease to keep surface condensation off your hardware!
Make sure there is something in the top of the container that won't hurt if it gets wet, between your hardware and the lid/seal...
That's so when you turn the container over to bury, and the seal leaks, your hardware will be in the air pocket in what was the bottom of the container.
Shop around some of the bigger Surplus Stores for containers, they are MUCH cheaper there than trying to build your own stuff from pipe and usually intended for moderately long term storage when you find them.
If you use metal containers, and if you bury something that is large or odd sized, you probably will use a metal container,
Remember to coat the outside of the container with basement sealer for exterior blocks or tar to keep the moisture and natural salts/acids from eating through the container.
Really lay it on THICK, may coats if you intend it to be there for long term storage.
If you own the land you are digging on,
Consider where you are burying things carefully.
Things buried under a side walk section on your own property is handy, secure, and you can be sure no one is going to dig that up without your advanced notice.
Things buried in a yard where the city or any number of utility companies can dig right on through might be a bad idea.
Sidewalks poured in sections are EASY to pry up with just a long pry bar and a fulcrum, and sidewalks explain why there might be a metal detector signal from that location...
Rebar or concrete wire in the cement.
Poured concrete pads for lawn decoration, like benches, water features, BBQ grill pads make good places if you want to spread yours away from the house.
Storage under places like the garage floor is a bad idea.
Lot's of hammering and hard to get to, and you will leave behind evidence that is REALLY hard to explain!
If you do want to store in the house, pour a slab next to a wall or footer (after you have secured your hardware) and then build work table or install cabinets over it.
Work table or cabinets will also restrict the weight on the area you couldn't pack, and keep the floor from cracking, and the cabinet/work table will cover the hole if you have to hammer up the floor...
If you don't own the land you are digging on...
Burying things next to roads is a bad idea, since that road might be widened and your stuff gets paved over,
Since every roadway has at least a 30' right of way from it's outside edge, stay well away from that 30' since any number of utilities will be digging through there at any give time...
MAKE SURE YOUR LOCATION LANDMARKS ARE PERMANENT!
I've heard more than one story about something being 'buried' and when they went back to get it in a few years, The tree or fence post or what ever they used to locate the 'Stash' was gone and they would up digging up half the county!
Rock outcroppings and concrete foundations or post are good landmarks, just about anything else can't be trusted!
Never trust a rock or anything else that can be moved!
I found that out myself when I was young (in the 70's)!
My Grandpa directed me to the cooker for a still they buried when the 'Law' got too close to their operation back in prohibition days, and before they went back to get it, probation ended so they left it burried
He was CONVINCED he could direct me to EXACTLY where the still was buried...
The old Oak they used as one of their markers was long gone, and the foundation stones of a barn were still there, but didn't look quite right to him...
So, after digging holes for about 2 months in all my spare time!
I finally found the still with a probe and dug it up.
It was more than 75 yards from where grandpa was "POSITIVE!" it was!
Did I mention that brass/bronze holds up well underground?!!!