Reliance makes some inexpensive containers as well. I found those at bigger sporting goods stores. They have a 1/2" pipe thread in the lid so I was able to replace their plastic valve with a laundry valve from a hardware store which allows me to screw on a hose.

A couple comments above brings up another issue, safely carrying things in the back of vehicles. Someone mentioned a cars trunk, but most of the car trunks I've ever seen the front wall of the trunk is simply the back seat of the car which means its just some thin strips of spring steel and seat fabric separating you from cargo in the trunk. That might hold a few bottles of water but as you start adding more gear such an an ax or shovel those could easily come through the seat in an accident. You need to secure things in your vehicles no matter what kind it is to insure yourself or others are safe. And securing means more than a bungie cord, those tend to snap when pulled too far such as in an accident. You can buy cargo straps for very little now a days and those should be one of the first items you get.
Even things such as screning a flashlight or fire extinguisher mount to the sheet metal with screws can pull out during the forces of a wreck. Likewise using wood to make cargo carriers, screws and nails can pull out. Always bolt or otherwise secure from the other side whatever you add in/on.