[jakobnev]

I think initially you get the best return from adding just enough deflector to the end of the roof on the Saab to prevent it from pulling the air down along the rear hatch. As you extent the deflector beyond that, you soon reach a point where it becomes better to start adding a partial boat tail to the trailer instead. As they are both extended they compete with each other with diminishing returns, until you have covered all the space between the Saab and have a complete boat tail on the trailer. (You are going to call it quits before that tho)

If rounding the front edges of the trailer ever comes into play, is difficult to say.

(This is all just in my humble laymans opinion btw.)