Just playing with the numbers, I made up a table & chart to calculate the co-ordinates for a boat tail, chart below:
Just wanting to confirm some things:
1/ The width tapering is based from the vertical centreline of the vehicle, hence this will usually result in a shorter tail calculation than the taper from the roof as half vehicle width is usually less than height. This will then result in a much more rapid reduction in width than the reduction in height.
2/ Do the side taper & roof taper both have to have the same 0 point? Because of the difference in length, the tapers are going to be out of sync at the first 10% interval anyway, as the sides will runout well before the top. I ask this because some vehicles may have more taper on the sides, or top which may already conform to the first 15-20% of boattail taper.
The profiles are calculated from the NACA 0039 profile I put up earlier, but the chart is not perfectly scaled for visual dimensions, just representative. I've used an interval spacing of 600mm (2').
So the way I'm looking at this, if I was to build a trailer cap, as long as I stayed within the sectional paramaters, at the very worst it would be "stuffing the wake" and at best some or all surfaces will re-attach flow and reduce drag.