The single biggest aerodynamic gain is with a full boat tail. So how would one do that?
Here's the science:
Here's a model:
This example uses a Gothic arch, but a truncated boat-tail could be four rigid curved trapezoids. The main thing is a rigid floor. With a high hinge line and short arms, it could swing down and align with the diffuser you're planning.
It's like the Trailer Tails on big-rig trucks, except it's not cut off to clear a loading dock.