A little late to the party, and Steve should be able to give much better advice.
But according to Kuchemann & Weber,
https://books.google.com/books/about/Aerodynamics_of_Propulsion.html?id=aRwIAQAAIAAJ
the shape along the last few percentage points of the length of the diffuser are counter-intuitive. They show a 'streamline' diffuser looking somewhat like the bell of a trumpet, where the duct reaches maximum area where it touches the face of the heat exchanger. (Apologies to Steve; redneck engineering terms.) They show the commonly occurring transitions, that go to full area at some distance away from the face, as having poor flow around the perimeter of the exchanger (making the effective area significantly smaller than actual).
(edit: This should say, 'poor pressure recovery'.) The book details actual pressure measurements in the two duct shapes. Here's the 1st link I could find to a comparison of 'normal' vs 'streamline' profiles; it's from a different author, but copies the relevant profiles and their pressure gradients from K&W. The drawings are on the 4th page of the document:
https://www.google.com/url?sa=i&source=images&cd=&ved=2ahUKEwj4uojF06jiAhUGmuAKHeovC7wQjRx6BAgBEAQ&url=%2Furl%3Fsa%3Di%26source%3Dimages%26cd%3D%26ved%3D%26url%3Dhttps%253A%252F%252Fwww.researchgate.net%252Fprofile%252FMarco_Pellegrini6%252Fpublication%252F253649721_The_bonus_of_aircraft_piston_engines_an_update_of_the_Meredith_effect%252Flinks%252F0046351fa125b4ad8a000000%252FThe-bonus-of-aircraft-piston-engines-an-update-of-the-Meredith-effect.pdf%26psig%3DAOvVaw2O55ms8IKsYHM75h-csxI5%26ust%3D1558390988386237&psig=AOvVaw2O55ms8IKsYHM75h-csxI5&ust=1558390988386237
Just a fwiw...
As Steve indicated, SCAT is very draggy; I really like the inner tube idea for a duct. Hmmm; I wonder if that would work instead of wetsuit material, for the James cowl inlet connectors...
I think it would be an interesting exercise for you to use Dan Horton's silicone impregnated fiberglass for the whole duct. Slick on the inside, flexible, relatively low temp environment to allow using inexpensive readily available silicone; what's not to like? And then you could tell those lazy folks (me) how it works.
Charlie