I did this, because I'm tired of
that being under my back seat, and it's way more accessible on those back country camping trips when it's just inside the rear glass and not under a pile of shit.
Not going to argue the legality of it, I don't really care, but if anyone else is looking for a way to mount
theirs...
This is a disposable shotgun (NEF Pardner Protector, bought it for $100 used, clone of the 870) in a
Big Sky Racks SBR-1 ($57.00 at Cabelas). Big Sky also makes an SBR-2 which holds two guns. I actually bought the SBR-2 because they were out of the SBR-1, but I simply didn't add the extra clamps on the back side for the second gun so this is basically the SBR-1.
The rack is very stout, nicely finished aluminum. If you look at the link, you will see that there is a padded middle section with a velcro band that is designed to secure the gun, I opted not to install that in the interests of keeping this nice and trim to the headliner. I have a fastex buckle holding the gun in and will use a short length of braided wire / cable lock to secure the gun against theft as needed. You could also build a aluminum lock assembly using aluminum plate and a clevis pin (see old pic, below).
By placing it here I was able to maximize two features: first, no drilling required. Just remove the coat hooks and bolt this up in their place. Second, it's accessible without opening the hatch if you roll the window down.
A few pics including from outside to give you an idea how invisible this thing is. I had one in my Xterra and no one ever noticed it, and it survived some harsh use in Moab and the like.
SBR « Big Sky Racks
I replaced the factory bolts with M6 x 25 button head cap bolts and star washers:
Alternative locking idea, from my old Xterra: