It's a quick and dirty freehand drawing; the proportions and many of the details are wrong (ignore the wheels and the joints).
The upper half will lower down over the bottom half (using a hand-winch and cables - not detailed in this sketch) to reduce the towing height. Both halves will have an externally-visible timber frame (probably made from seasoned european oak with proper woodworking joints). The frame infill panels will probably be made from western red cedar T&G or shiplap. Some of the upper panels will contain fixed Perspex windows instead. My current thinking on the roof covering is that a sheet of polythene DPM (for waterproofing), then aluminium foil (for insulation), then vinyl flooring (a single sheet over the whole roof), and finally a coat or two of bituminous roof paint might work well.
I've since done a drawing that has the correct proportions, but I want to redraw it before I post it up because I've changed several important details.
