Hi Mungo - answers to questions as follows:
I tried 2 sized beds in Sketchup, and the king single is probably the best fit. A std queen size will fit (you will find my mock up of that off to the side of the canopy in the SketchUp file) but doesn't give very much room to actually get in/out of bed.
No gas. All appliances will be Coleman dual fuel. I have an Engel vertical fridge, so that will be battery powered. Am planning to cook inside (vent with small fan for fumes/etc).
The dimensions of the canopy are as is. I don't want to modify it, it would be too much work. I could extend it upwards a bit, but the floorplan size would require major re-work.
The opening at the front of the canopy will lead into the cabin. I will remove the rear window of the LR cabin, and cut the back to make a sufficient size opening, although I can see a bit of crawling might be required! To join the canopy to the cabin, I will make up a flange and join with rubber strip to allow canopy and cabin to have a bit of movement.
Material - probably aluminium for weight. Rivets/Sikaflex to hold it together. I've got a TIG welder, but am concerned that corrugations + welded joints may come adrift.
The 'box' underneath is a hugely long drawer, in a separate compartment. I will need to fashion some way of getting things out of the end of it, in drawers probably. I have a couple of waffle boards that will go down there. Am thinking that all the 'dirty' stuff will go here, like snatch straps, oily spares, campfire tripod, etc.
I want to put a water tank underneath the canopy. I have enough space for around 120L, but might divide this into 2 compartments of grey water and drinking water.
http://gestaltn.ipower.com/Defender/CanopyOff130.JPG
Any more question, just ask away :)

