Underground and dry, where it isn't easy found outside the house is unbeatable storage conditions. Lead bullets will retain hardness longest, lube and powder lasts longest in cool areas. Dry is mandatory to prevent primer and cartridge deteroriation.
Wouldn't hurt to dry that dissicant at least every 6 months. This way you'll know if moisture is beginning to get in before it does without you knowing and takes it toll.
A very neat and absolutely water proof container can be made with PVC pipe and cemented caps on the ends. Easy to bury without a lot of disturbance to the ground. Leave room so the contents will not be hit when you saw one end off. Dissicant inside won't hurt, but isn't needed if everything is dry to start. I stored some special papers this way, under water, for I think around 10 years. (Not many people dig around looking for things under water!) The papers were as new when the pipe was opened.
Be liberal with the glue, and be sure the pipe ends are smoothed so they don't strip glue as the cap is slipped on. Once the cap is on, twist it a little so the glue can settle in around any dry spots. Use schedule 40 PVC for best life. ABS sewer pipe is as good, but only available up to 4 inch inside diameter. PVC isn't much more expensive, is available up to 12 inches diamete, at least, and VERY available in 6 inch diameter, Ammo cans rust out if there dampness, while PVC is invincable to water, dirt etc. That's what it's made for.