Occasionally, I print large photos (20x30). What kind of container should I use if I want them to last approximately 20 years?
Note: Assume I am not concerned about fire.
Archival cardboard storage boxes ought to do the trick. They may be difficult to find in your size at a photo store (even B&H only has them up to 20x24-1/2"), but you should be able to get them at any large artists' materials shop (perhaps as a special order) or online through a museum supplier. If worse comes to worst, you can construct the boxes from thick (at least 4-ply) archival mounting board and metal box corners.
You'll also need archival low-abrasion separation sheets (like glassene, but acid-free) between the prints. A double layer of separators -- a slick sheet on the print surface, and a more absorbant, thicker paper between the slick sheet and the back of the next print -- would be better.
The cardboard and the thicker separation sheets will moderate humidity. The box will keep out the light. Using archival materials means you won't have to worry about acid damage from the container or any outgassing that may negatively affect the paper, coatings or pigments. That about covers the requirements -- and your prints should last significantly longer than twenty years under those conditions.
Both plastic and metal containers can be problematic. Some plastics are safe, but it's hard to find safe containers in the size you need, and next to impossible to build 'em if you can't buy 'em. Metals can corrode silently. They may be useful as outer containers for protection from physical damage, but you should still be using cardboard inner containers to prevent direct contact with the metal.
my parents stored our baby photos in shoeboxes in the bottom of the bookshelves and in the attic, they were still quite good when we found it again some 30 years later when they sold the house and moved into town (from living out in the woods).