Well, I played with your STL file. I know of several ways to accomplish your goal, so I thought it would take me five minutes to test and verify. Instead, after significantly more time than five minutes, I failed. I tried playing with positive space and negative space. I tried in the solid workspace and the surface workspace. I tried with combines and with boundary fills. Note that all these approaches have worked multiple times in the past on object created within Fusion 360, so there is either something āwrongā about the mesh, or there is a bug in Fusion 360.
The issue concerns splitting objects. For example, normally when an extrude/cut is executed, if the mesh is separated, then multiple objects are the result. But with your mesh none of the operations I tried resulted in multiple objects. In my tries, I either got a single object or the operation failed. There was a line on the mesh where the two halves should have been bifurcated, but still only one object. It was like the cut operations somehow repaired the split between the two halves to keep it one object.
So, when that all failed, I went looking for a hackā¦and I found one. It is ugly and imperfect, but it may get the job done. The idea is to first create the cavity. Then to create a series of planes starting at the tip of the deepest part of the cavity and working up. At each level, a sketch is created and a Project/Intersect of the Body is done. The resulting sketch is then used to Extrude/Cut a piece of the overhang away. It only took me about 10 minutes to cut the overhang away, but I know the result is imperfect. Using planes closer together would improve the result, but I donāt think you can get a perfect result using this technique.
I tried to upload the Fusion 360 file so you can see the timeline and the result, but the file is too big for upload to this forum. Here is result as an STL:
Trim.zip (84.4 KB)