Depending on how much room you have, I like the
Rubbermaid Action Packers I have several of these in different sizes setup depending on the contents and need. I have an Astro Van so there is a ton of room in the back since I eliminated the rear passenger seat long ago.
One is setup with all of the items I may need when we get a SAR call, and another one is setup with raingear, boots, towels, etc. The nice thing is that if the raingear gets used, all of the wet gear can go back in the container for transport home without making a mess in the vehicle. I also have one that is setup with all of the fluids, basic tools and breakdown stuff for roadtrips that stays in the vehicle 24/7. These come in varying sizes. Best price I have seen on them is at Wal-Mart, although sometimes Big Lots gets them at a decent price. When not needed, they stack nicely in the garage and I can just grab and go as needed.
The Action Packer can be padlocked, and if you wanted to get creative, you could bolt it into the vehicle if you are worried about it being stolen out of the vehicle. If bolted in securely, someone would need a pretty beefy tool to cut through it to get to the contents.