If you've got room under the window glass (when the window is rolled all the way down) you could cut out a rectangular hole in the inner door structure. Then measure and cut a flat sheet of 22 gage sheet metal and bend the four corners and mounting flanges to fit in the hole cutout you made. I suggest mig-welding the corners for rigidity and lining the inside of your pocket with felt at a minimum (you may be able to select felt colors from some fabric outlets or ask you local auto interior upholsterer for samples he might be able to provide).
The only thing I havent figured out yet is what to use as trim to hide the door upholstery panel cutout. Also, mine had a hinged door (with hinge at bottom and a simple latch at top with small knob to pull down). It would need a similar corner trim around the door edges.
You could susbstitute the door with a pocket design that extends down into the bottom of the door, but the 20 gage metal box design would be more elaborate, but certainly not impossible.