I have a few ammo cases laying around and one particular seemed like it might be a good fit behind the back seat. Sure enough, the cannon ammunition crates are a perfect fit.
I had to trim a very small piece of the plastic guard for the third brake light, but once that was done it fits perfectly. It WILL NOT come out with the tail gate closed, and it cannot be opened. And it will keep the contents dry.
Plenty big for my tools, gloves, recovery gear, etc.
If anyone local (SoCal) would like some of these, I can get more for $10 each, or just send you to the source.