A lot of very useful military surplus gear is made from cotton canvas. Canvas is rugged and depending on the sample, may be somewhat naturally water repellent. When wet, the fibers swell and keep out more water.
However, if you expect it to be exposed to drenching rain you'll want to treat the canvas with something to make it more water resistant. Traditionally, this was done with a variety of substances, ranging from boiled linseed oil or beeswax, to alum salts or parafin.
One waterproofing treatment which has been available over the counter for several decades is Sno Seal. Its main ingredient is beeswax and it's been sold for the purpose of waterproofing leather. I've been using it for years on stuff like my boots and leather possibles bag. It's also good for use on holsters because it does not soften the leather. Last night I decided to give it a try on some canvas.
A few years ago I bought this canvas shoulder bag at a gun show. It was marked as being a Czech bread or gas mask bag.
It's a nice little bag suitable for carrying some stuff on a day hike. Here's what it looks like now that I've given it a coat of Sno Seal.
The Sno Seal slightly darkened the canvas and gives it a waxy feel. If you look closely you can see some beads of water. After waxing it I held it under a runng faucet for about 20 seconds. The water ran right off.
If you want to try this I suggest applying some Sno Seal onto a small, inconspicuous area of whatever you're trying to waterproof in case you don't like either the color or feel.
While modern materials have many advantages over traditional outdoor gear, the older stuff still can work well.