"Tuffy F-150 Under Rear Seat Lockbox" - UPDATE
Thought I would report back with 1st-hand experience, and leave prior thread comments intact. Hoping you find this update useful, and that I don't get in too much trouble for reviving an older thread....
I did an extensive online search for a way to secure my firearm, when I wasn't able to legally carry - i.e. trips to the Post Office. I also wanted to keep stuff from sliding around on the rear floor of my F-150 Supercrew.
Advertised with 16-gauge steel, it will likely deter the average smash-and-grab thief, now that the truck alarm is blaring in his ears from the break-in. The guy who has some time, the right tools, and the ability ride out the alarm can get in, no question.
If successful prying it open and/or breaking one of the locks, though, he'll now be faced with a V-Line pistol "case" inside (I refuse to call it a "safe") if he wants my gun. The V-Line is cable-locked to a bracket secured under the rear seat, so that should slow him down even more.
Pic 1: The finished install. A neat and elegant solution that keeps stuff from sliding around on the rear seat floor. Completely hidden from view until one of the rear doors is opened. It does not alter rear seat passenger comfort in any way.
Pic 2: See the cable locks in the lower right corner of the picture? --- I finally found a
good way to use those things! The lockbox stretches across the width of the floor, and has two locking lids. Yes, I did give up a flat rear floor, but I was willing to make that tradeoff. And if I had to, I could always temporarily "uninstall" the lockbox by removing the wing nuts (see pic 3 below) and unlocking the cables, and regain that flat floor.
Pic 3: And now you see what those cable locks are for - to secure the V-Line "case" to the seat bracket. The gun in the picture is my old, trusty Winchester Defender riot gun, there solely to provide some scale for the picture. (At all other times, it is securely stored in my real safe.)
Pic 4: There is quite a bit of room for item storage as you can see - the black box on the far side contains 2-gauge jaw-to-jaw all copper jumper cables. In addition to my gun, I'd be pretty ticked if a thief broke in and stole these from me!
I have been using this lockbox for the past couple of months now, and am pleased all-round. Very easy to get in and out of. I purchased it directly from the factory, where it was backordered. Took about 4 weeks before the next production run filled my order. My signature on delivery was required, and that necessitated a special trip to the UPS office (where I carried, of course, btw...

), but that was the only hassle. Install instructions were easy to follow and the process overall was simple.
Works for me. YMMV.