40 CFR 86.129-80 Road load power...
 

For Aerohead et al: https://www.law.cornell.edu/cfr/text/40/86.129-80

...especially this graphic & table:

http://www.ecfr.gov/graphics/er06oc93.161.gif

Table I—Protuberance Power, P. Versus Total Protuberance Frontal Area, Ap

Ap (sq. foot) P (horsepower)
_____Ap <0.30 0.0
0.30 Ap <0.60 0.40
0.60 Ap <0.90 0.70
0.90 Ap <1.20 1.00
1.20 Ap <1.50 1.30
1.50 Ap <1.80 1.60
1.80 Ap <2.10 1.90
2.10 Ap <2.40 2.20
2.40 Ap <2.70 2.50
2.70 Ap <3.00 2.80
3.00 Ap ______3.10


NOTE: CFR = Code of Federal Regulations.

protuberance
 

It looks like we could construct a Cartesian grid from the posted data,from zero,to 3.0 sq-ft frontal area of the greenhouse,then extrapolate for larger 'roofs', picking off their 50-mph road load from the grid.
Months ago,I was working on a side elevation drawing of this illustration,which included the wheels,to evaluate the aft-body contour based upon overall height.
It's buried somewhere in the rat's nest.:p