The ammo can idea could work. It should be bearproof. Chaining it to a tree should also help keep it from being dragged off. Your basic approach seems fine here.

Some further tips about caches.

Placing a cache near a road works best, if that option is available to you, simply to limit the distance you have to carry it when you set it out and when you retreive it. An ammo box is not light and is pretty awkward to carry very far.

You'll want it to be entirely out of sight from the road or the trail, as in unnoticeable. Just setting it behind a convenient fallen log could be enough of a hiding place. Covering it over with large debris is also good.

Take some time to think out the placement and how you'll recognize the spot when you need to find it again. GPS would be plenty good enough, but if you don't have one, leave an unobtrusive marker, perhaps using natural materials arranged in a somewhat unnatural way. If you spend several minutes to memorize the placement, you should discover yourself walking right up to it without a moment of hesitation.

Make sure it isn't in a place where it will overheat in sunlight.