The stacks shown in the second pic of the OP look like they might do some good. What you show in your last sets of pics looks like it would interfere with the flow. But I'm not an Aerodynamics Engineer (or Engineer of any sort). I'm looking forward to seeing your results and hope that my guesses are wrong.
A big
dowel pin reamer may give you the taper you are looking for but not sure how it would work on plastic.