Hmm, well, I do most of my bush stomping is for pleasure not work, so we're talking late spring to mid fall excursions (hypothermia not an equation generally) and with a canoe and it's associated pack.
The "Oh sh1t" pack is perhaps 3 liters in size and is designed to help if I become separated from the canoe (or have damaged the canoe to the point it becomes useless) and I'm forced to "walk out".
You'll have to design your own for the area you're in, but there are a few items that are in my opinion universal: Map of the area and compass (the map is protected from getting wet), fire starting, (I have several butane lighters, waterproof matches, flint&steel, and tinder, candles (2) as well as store bought fire starter "sticks", fatwood if you can find some.), stick "pop" flares, light sticks (snap and shake), magnifying glass, small tarp (I carry 2 emergency blankets, dual use), fishing line, hooks, weights, snare line, and a store bought sling shot rubbers and pouch (multipurpose as a {gulp} tourniquet.) Cut you're own "Y" stick and voila, birds.
Knife; I carry 2 in the oh sh1t bag, a large Victorianox with the gazillion tools on it...you know the type, and a leatherman. These are quite secondary in design as I always carry a belt knife when in the bush and assume that when sh1t hits the fan, I'll still have the belt knife.
So far what is incorporated will take care of fire, shelter and the ability to eat (which is really the least of my concern) and signalling for rescue if necessary.
What is not yet addressed is the alpha problem...water. We all know there's lots in Ontario, however it isn't safe to drink, or may not be safe to drink, and it would be near impossible to walk out of anywhere with a fever while sh1tting yourself with violent diarrhea. I carry a filtration system in my main pack but that maybe gone... so, in my oh sh1t bag you will find iodine (again dual purpose) and water purification tablets with a collapsible steel cup. (invaluable item, hard to boil water without a container of some description) I also take antihistamine tablets (negates body's swelling reaction), aspirin (when dehydrated, the accompanying headache I find nearly debilitating...and you will become dehydrated). Another universal item most don't think about is duct tape, put a roll in there. It's relatively light and has a million uses from securing splints, holding lacerations closed, to repairing a hole punched in that canoe I mentioned, signalling, shelter, an on and on, and on...
It requires some thought on your part, the pack is not "an everything + kitchen sink" type of thing. It is designed to be light (or you won't carry it negating it's benefit) and have multipurpose items that will last several days (a week?) and nights. If to walk out would take longer, it may be a better idea to stay put and wait for help. (which is on it's way two days after you went awol because you told someone where you're going and when you expect to be back...right!?

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