Aside from the obligatory knife and para cord, this is what I would fit in that ammo box.

-Headlamp

Shelter/fire:
- Tarp. Find a cheap one wherever you can. Just make sure it's large enough to cover you and has metal grommets.
- Poncho and space blanket. Space blankets aren't made for insulation, but they can block the wind and reflect radiation, which is useful for fire shields. Also, they don't take up much space or weight so there's no point in not bringing it. Just don't make it your only tool for staying warm.
- Bic lighter. They don't break as easily as the cheap kind. If you want to practice with firesteel though, definitely bring some along. The lighter is just a better first option.
- (optional) Fire starting tablets. They're not totally necessary, but they're nice in a pinch and they give you a long flame in case it's wet out.

Water
- Small stainless steel water bottle with a carabiner. For boiling and carrying water obviously. Pack items in the bottle to conserve space in the ammo case.
http://www.amazon.com/Klean-Kanteen-Stainless-Bottle-Sports/dp/B009W7F75U/ref=pd_bxgy_sg_img_y
Here's a 12 oz one as an example. You can probably find a similar size at Goodwill for a few bucks.
-Iodine tablets. In case you can't get a fire going, you can have potable water in about a half hour. Just bring some powdered electrolyte drink like Gatorade to cover the taste.

First Aid:
-Bandaids
-Alcohol wipes.
(optional) gauze and a small roll of medical tape. The tape can actually be really useful in other ways. Get creative.

Travel:
-A compass and small pencil to mark the map.

Other if space available:
- A pair of socks in a plastic bag. You never know when a nice dry pair of socks could come in handy. Plus, you can turn them into a makeshift water filter for dirty water.
- A small roll of trash bags with ties. Many uses. Doesn't take up much space.
-Spork

Edited for fomatting.