I decided to add some expanding foam under the bonnet. Since I was taking the car out for a spin, I decided to tuft test the boattail, to see whether my design looked up to scratch or not.
I could see much better in the car than my camera could (due to the motion of the tufts). My impression was that every tuft along the boattail was travelling parallel to the plane of the plastic it was attached to, in the general direction to the back of the car. There was one that was oscillating about 180 degrees in that direction, but was still parallel to that plane at all times.
I'm not sure exactly the reason for why that last tuft acted that way, but even then the average direction it pointed was the correct way, and it wasn't away from the surface, so it doesn't look like a separation bubble would (looping back and around). My wife's car does that (tested, no photos), and the angle of the hatchback is steeper.
I stuck a tuft right on the end, and that tended to curl back around due to the lower pressure under there. Which is what we expect - an area of low pressure but smaller than it otherwise would be.
Of course, the tuft testing is only a guide, the proof is in drag coefficient, which seems pretty good. Of course, I haven't tested it with various angles etc. I only have so much time.