Soooo.... I had ordered a TuffLoc rifle/carbine lock for my AR15 to mount under the back seat so it is secure, yet accessible...
I canceled that order today, because after a significant amount of research on what that would entail (I'd pay $111.00 for the actual lock, then ALSO need to install an L bracket at the butt of my AR to ensure the TuffLoc would actually secure my AR....)
I deducted that two pieces of 1/4" thick steel and a good lock, properly applied, would serve the same purpose...
Soooo, without spending $111, and still having to install a piece of angles steel to snug it up to prevent it from just being slid out of the cuff, I found a very good way to properly secure an AR15 under the back seat, with two pieces of 1/4" steel and a good lock...
I made an "L" bracket of 1/4" steel (1" wide) six and 3/4" long, bent at a 90 degree angle at the 4" mark, so the long end is 4" and the short end is 2 3/4"... Made holes in it to mount the long end to the truck body with self-tapping screws. I made three holes for this and I used 3 self-tapping screws: #12 x 1 1/2 inch. This will ensure the base bracket is solidly mounted to the truck. I mounted this about 3" out from the seat release levers.
To ensure the mount will not mar my AR15, I triple coated the mount with 1 3/4" heat shrink .... the triple coating of the heat shrink ensures that the mount itself will not mar my AR.
The "L bracket will surely secure my AR within the trigger housing, however, there is always the possibility of somebody using a small punch or even a paper clip to release the lower bar of the trigger guard...
To prevent such an override, I made another "L" shaped piece of steel to act as a trigger guard shield to slide over the lock mount. This will ensure that the trigger guard cannot simply be bypassed with pressing the roll-pin or trigger guard release pin.
With this 1/4" thick steel shield in place, there is no way to override the locking mechanism and release the AR15. It covers the trigger housing, and angles down to the underside of the AR15, so nothing can get in there to depress the bottom section of the trigger guard pin.
This mount was designed and measured for a SBR or Pistol AR15 with a 7.5" barrel, however, it also works for a full-length AR15 (see photos), however, the caveat is, with a full-length AR15, the 17" toolbox on the right side of the passenger seat is not feasible. It just won't fit comfortably.
While a full length AR15 absolutely fits, the toolbox area normally available under the passenger side seat is shortened, and I could not fit it in there with the length of the barrel of a full-length AR15.
Bottom line: You can easily secure an AR15 under the back seat, however, if you ALSO have the 17" toolbox secured under there, you'll need to make sure your AR15 is short (7.5" or 10'5" Short-barreled Rifle or pistol with an approved Sig brace pistol stock (SB15 or other approved pistol brace).
Once you have that measured and mounted, you'll want to ensure the steel bracket won't mar your AR, so either wrap it with electrical tape or heat shrink 2 or three layers of heat shrink tubing on it (I used three layers of 1 3/4" heat shrink tubing...worked perfectly)
Then, cut a 2.5" x 2.5" piece of steel, bend it at the 2/5" mark to a 90 degree angle, and grind the area to conform to the trigger housing to size...(you want to ensure the metal fits nicely inside the contour of the trigger housing of the AR) and cut a 1" slot to allow for the mount piece to fit through (this part might take the most cut & measure trial).
After you bend that piece of steel (I used my vice and a hammer on the workbench for both pieces) and have the slot cut and felt (or whatever type of protective material) backing applied, put the AR down with the trigger guard over the upward facing mount bracket, and slide the trigger guard bracket over that, and affix the lock. I used a Master lock, but considered a "puck lock" (they cost about $20 on Amazon, and while more secure, they offer less wiggle room for the size of the bracket).
This works very well for ensuring your AR or other carbine is safe and secure under the seat, with fairly quick access at the same time.
I played with this for a few days to get just the right position and measurements. I'm installing the same exact mount bracket in my wife's Xterra (but this time, with zero "trial and error"). It works great!