I've still been fine tuning the spreadsheet.
Basically I have the x coordinate being tapered by side width, and y coordinate being tapered by vertical height.
I found the overall vertical profile looked too aggressive because the side profile is based on a much smaller dimension, at the tip of tail it was approaching 45%, depending on cross sectional profile so I had to introduce a factor to reduce the rate of side taper.
Firstly I entered the dimensions of vehicle, to produce a theoretical boat tail, then estimated the lengthwise position where trailer canopy would be widest, took the dimensions from the vehicle profile section at this point (5th one in image in black).
I entered those dimensions into another sheet made some small adjustments to get a more appropriate profile for the trailer canopy. to get the image below.
It is fairly conservative, the absolute steepest point the vertical centreline at the rear tip is estimated around 20 degrees, the trailer canopy starts with the boattail curve, so very gentle to begin with, this should support reattachment of air from vehicle. Utility also played a significant role in the design ideas, I figured a good usable space that helped aero a little was far better than a space that didn't work functionally and maybe didn't even help aero.
Basically the front of canopy will be well radiased down to the drawbar, the front interior of load area will be about 1,200mm sq (4 ft), the rear will maintain rectangular tailgate, 1,200mm (4ft) wide, 350mm (14 inch) high with a hemisphere above, that will take max height to 800mm (2ft 8") at rear.
Still thinking about construction methods as this may also have some bearing on the final design.