Quote Originally Posted by fred_dot_u View Post
Your best shot would be to sign up for the free hobbyist license of Fusion 360. I am able to load the .STEP file into F360. It should be a reasonably straighforward task to strip away each component and export as STL. I won't say it's going to be easy, just a simple set of steps to accomplish the goal.
That sounds like a lot of work... Isn't it possible to close the outer surface automatically somehow? It doesn't have to be perfect, it's just for a test fit in my setup.