A few months ago I decided to get a truck shotgun. I figured the under-seat storage in the back of my 2011 double cab would be a great place to keep one. The storage compartment is obviously limited, but I found that the stockless Mossberg 500 Cruiser fits nicely. (I doubt you'd be able to get anything with a stock in there comfortably.)
Of course, just letting the gun lay lopsided in the container wasn't very aesthetically appealing, and over time it sliding back and forth would wear both the gun and the storage compartment. So, I devised a simple, cheap mod to safely secure the gun inside the compartment using duct tape's friendly cousin, Velcro.
* 2x #10 1/2" machine screws
* 2x #10 steel washers
* 2x #10 nut
* 2x #10 black plastic screw covers
* 2+ feet double-sided Velcro
* Drill w/ 5/32" bit
* 2x flat head screwdriver
* Adjustable wrench (a 10mm socket should suffice for our purposes)
* Marker pen/chalk (preferably a light color)
Step 1: Remove the under-seat storage compartment
This is accomplished by prying up the nine black fasteners along the front and rear edges of the compartment using two flat head screwdrivers. They may seem stubborn at first but with a bit of effort they will eventually pop straight up. (There's probably a tool designed specifically for this, but damned if I know what it is. A very small claw hammer might work as well.) I suggest sealing the fasteners in a plastic immediately to so that they don't disappear.
With all nine fasteners removed, the plastic compartment lifts right out.
Step 2: Cut two strips of Velcro to length
Wrap Velcro around the two points at which you want to secure your gun leaving plenty of extra slack and cut two strips of sufficient length. You can always trip them down later if necessary.
Step 3: Drill two holes along the front of the compartment
Now, the ideal placement of these holes will depend on what gun you have. For the 500 Crusier, I drilled two holes about 3/8" below the lip of the container: one about half an inch from the left inside edge and the other almost directly underneath the third fastener hole from the left. You'll probably need to experiment with the Velcro and positioning of the gun to come up with the best spots.
A word of caution here: Make sure the placement of the holes doesn't interfere with the two slots for the locking mechanisms of the compartment lids. Also avoid drilling the holes close to the vertical support on the outside of the container as they will prevent the washers from laying flat.
Step 4: Putting it together
Fold each Velcro strip at the point you want to attach it to the compartment and cut a very short slit vertically across the fold. This slit should be just long enough to force the screw the Velcro and no longer. Keep in mind that you want the overlapping portion of the Velcro loop to be above the gun, which should be the gun's right side.
Put a screw through one of the screw covers and then force it through the Velcro strip with the loop (soft) side facing the screw head. Drive the screw into the wall of the storage compartment (head facing inside) and secure a washer and nut on the end of the screw. Repeat for the second screw.
Again, the order of the component from inside to outside the compartment is: screw head, screw cover, Velcro, compartment wall, washer, nut.
Once the nuts have been tightened sufficiently, place the storage compartment back in the truck and replace the fasteners. Now you should be set to mount your gun.
I've found that if I keep the right-side loop just loose enough, I can very easily undo the left-side loop and slide the gun out without lifting the right side of the compartment (under the larger half of the back seat).