I'm looking for a box I can carry in the back of my truck to store things (guns, groceries, camping gear, etc.) in temporarily while I stop places to reduce the chance of those items getting stolen or exposed to the weather. I'm not looking for Fort Knox, just something to stop a opportunistic thief from walking by and grabbing an item out of my truck bed. So, ideally, something that is semi-rigid and lockable, and difficult/conspicuous for one person to try to carry off on their own. At the same time, I'd like something that when empty I can easily remove and toss in the garge. Teh closest thing I have come to finding is the Plano 1919 Sportsman's Trunk. The only issue I have with that is it's not quite long enough. I need something that is at least 45" x 24" x 12" inside. Any suggestions?

Jobox makes some serious truck boxes.  Thieves love construction trucks and the tools inside; the Jobox seems to stymie all but the most dedicated.  It sounds like you're looking for something less serious than those, but the price of one gun, and the piece of mind you'll lose knowing someone with potentially bad motives has your gun, makes the investment in something tough a little easier to swallow.

I checked your local craigslist (https://phoenix.craigslist.org/search/sso?query=jobox&sort=rel) and it doesn't look like much in your area available.  You can expand the search by looking for 'truck tool box' or 'truck toolbox' etc.

Be careful with the plastic boxes- some of them are too easy to pry open with a screwdriver.  In any event, think like a thief when you look at them.