Hey, Mike-
Here are a couple of thoughts for you to consider, if they haven't already occurred to you.
While it's probably more expensive than dealing direct, I bet Alembic can have one made for you through their dealer and nail the size/shape dead on. Certainly, quality would be beyond question.
I believe there are different grades of flight cases as well, so you probably want to make sure you get something extra protective. You're going to have a lot of surface area on the sides with that big bass, so I wonder if you need some thicker plywood for the strength? Do those cases have any sort of internal support frame to support the sides in this cse, or is it all on the thickness/stiffness of the sides plus the foam?
For that monster, you probably also want to consider one of those cases with wheels on the bottom.
-Bob