Custom Bed Box idea

So, stop me if you've seen something like this before. My friend seems to think one exists, but I haven't found anything like it, plus, I don't even know what it would be called.

Here goes:

So at work there's the occasional need to haul plywood and drywall. Not super often, but often enough. At 4'w, it'll lay across the wheel well bumps in the bed, but won't be supported. Sure, I could angle them, but it won't always be just 3 or 4 pieces.

First thought...build up some support out of scrap 2x4s to lay down when I'm loading anything like this. Then I got to thinking that I'd always have these 2x4 support things laying around in my bed.

Second thought was that I'd like to keep some tools in my truck. Since I'm in the city, the only answer is a lockbox. The typical lockbox--to me anyways--just looks like it'll invite someone to pry into it, knowing that there are likely tools in there. And besides, I'd still like the clean look of a bed cover.

This brings me to my idea. A homemade--unless purchase-able--lockbox that fits in the bed of the truck and rises no more than the height of the wheel well bumps. I'd like it to go from side to side and then end right about in the middle, if not a little past the wheel wells.

I can incorporate all of the locks, lids and potentially even a roll out drawer that I want. Now, I have bed support for any of the times I need to haul 4' wide materials, a lockbox AND it's hidden out of sight by a locking cover.

Sheet metal, rivets, maybe some diamond plate for the top, waterproofing/weatherseal and bolt it through the bed of the truck.

Now...
A) does something like this exist already?
B) pick apart my idea...what am I missing and not thinking of?