A good knife, I mean GOOD knife, not that it means expensive. If you have room, a 4 inch fixed knife blade would be ideal, the biggest I like is my US Air Force Survival knife but a Buck 110 is good too, the only folding knife I would use for it.
Matches, UCO strike anywhere variety, a ferro rod, some cottonballs soaked in vasoline, a small first aid kit, some water tablets to purify water. Some fishing line, a few hooks and a couple split shot sinkers.
An emergency blanket, some rope, jute cord works good. A couple of black trash bags, the big ones, they make a halfway decent poncho if need be.
A compass, preferably two.
A small multi-tool.
I can carry a pretty decent sized emergency kit in my shoulder bag or even in a pouch on my rifle. You don't need a huge box, you'll find if its something you don't want to carry, you're likely to forget it or just get sick of it. Ditch the box and put it in a pack or make it more portable.

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Vaya con Dios