Kids and teens go thru al ot of stuff. They may change bags and daypacks frequently whebnever something more fashionable comes in the market. That means an endless "supply" of containers for all imaginable purposes.

As an example, I use a discarded soft lunch box for a mini-tool kit for quick fixes around the house.

For a cache, you can choose one or two day packs or shoulder bags and put in the cache. You may need it for any possible purpose. For example, you are at work and evacuation is announced, you quiclky call the wife to meet you somewhere and go to school to take the kids. Thge family meet somewehre and the kids have NO bags to carry emergency stuff. When you open your cache, you can distribute the bags.

Or suppose you have your bags, but you need the added insurance of MORE supplies. So you take a shoulder bag from the cache, stop at a conveient store and fill it up with supplies.

Next time your kid or teen throws a bag or pack in good condition, take it and put in in the cache.