Once you own an action packer, those SWAG mounts pay for themselves. I have one in the back of my pickup that I keep all my every day "just in case" crap in like oil, coolant, spare belt etc.. The first action packer I had, I used the tie down loops under the handle to secure the packer to the back of the bed. It took a couple years, but they ultimatley tore off one side rendering the red lid lock useless. I bought another one and this time I use a strap around the body of the packer to hold it against the back of the bed and it ended up distorting the box making the top an ill fit. Ended up letting it sit in the sun for a day with the lid shoe horned on and the top seems to fit better. This is my daily driver, if this was something bouncing over rock down a trail, I do not think it would have lasted all that long.
That said, in my jeep, I use a steel surplus mortar case that is wide and short. I attached a lock mechanism to give it a positive, secure lock.
edit, this is the exact mortar case I have
http://www.ebay.com/itm/US-Military-Surplus-Empty-81mm-Mortar-Ammo-Can-Tool-box-Storage-Chest-Vintage-/371509945885?hash=item567fb7e61d:g:RKsAAOSwnH1WZx-F