i used to employ a very similar design for transporting all kinds of stuff i wasn't supposed to have. never thought to incorporate it for legitimate gun use, good thinking outside the box (pun intended).
For secure transport and in vehicle storage purposes this is a handy and cheap idea for pistols too.A Toolbox "Low Profile" M4 Case
by Cincinnatus of AR15.com
Here we go, step by step
1) What you need:
30" Lowes toolbox
thin rubber pad
one layer of 2" dense grey foam
one layer of 3" dense grey foam
Electric carving knife or Box Cutter knives (with the break off blades)
The Foam can be bought at www.canalrubber.com or most hobby or sewing stores.
2)Take the thin rubber, and pad the bottom of the box
3)Take the 2" foam, and place your lower onto it.
... <You get the idea but click the link below for complete directions toward the finished product>...
11) Place the 3" foam in the box on top of the 2" foam. and make sure that everything fits as expected.
12) Once you are happy with the fit, remove the foam pieces and glue them together with a spray adhesive like 3M #77 spray cans S/B available at Home Depot. When done you can reassemble the toolbox and use the tray as a cover!
Other comments from Cincinnatus ...
The Upper and lower will not touch due to the differences in thickness in the foam pieces. If someone intend to store (not just transport) a rifle in such a box, It is advised that you put in a desiccant pack/device.
The article can be found at http://www.quarterbore.com/ar15m16/toolbox.html