I'm estimating:
*about 57-inches length extension from hatch gap at top.
*8-degree down-slope (no separation and good rearward visibility).
*around 3-degrees diffuser up-slope.(okay)
*Festiva appears to be raked quite a bit (nose down,tail up).This would keep the entire roof top in attack,right up to the gap.Kinda like Probe-IV when dropped for highway.
*Some type of former might be helpful for inside the very back to reinforce the sheetmetal,prevent deformation.
*Yes,at least a plan-view photo would be nice,for a look at plan-taper of tail,and rear 'transom' area.
I like it
PS,'like the 56-mpg too!