Here are a few ideas from a local company here is South Africa. We have quite a few of them manufacturing systems for the various 4x4's on the market....... but, damn expensive, so I'm in the process of designing my own.
2 drawers on the bottom (which take up the entire back door), 200mm high, with the fridge/freezer mounted on a slider on top of that.
MAterials of construction:
1,2 mm thick galvanized mild steel plate bent in a box bender to the required shape.
There are 2 systems used for the sliding mechanism for the drawers: either bearings which run on a bent-up square section of the galvanised plate, or heavy duty drawer slides.
Marine ply top.
Entire drawer system is covered with carpet which has been glued on with contact adhesive. Especially the inside of the draws to stop the goodies inside from rattling.
I will be using these materials with drawer slide as apposed to the bearings.
As for the 12 inch sub...... I have removed the rear door liner and am in the process of making engineering drawings to machine a 3mm thick aluminum plate, on my CNC router, to mount there. Will be powder coated light beige to match the rest of my interior. This provides a great sable substrate to mount stuff on.
I will be posting the install on the tech section once I have finished.....and yes I will e-mail any interested parties a DXF file for machining if they want...
http://www.panelbeaters.co.za/packing_sys.html