I want to have a rectangular solid with a cavity which is enclosed by multiple surface imported objects.

What I have right now is a model that has two major parts, a ultrasound probe case and a fully filled rectangular box. The ultrasound probe is made up of 21 solid bodies,  two hundreds surface imported and it is actually imported from .stl. The rectangular solid is added by me after the import. It is "Surface-Knit2" in the model.

The aim is to get a rectangle solid with a cavity with shape of ultrasound probe. The plan is to remove the solid enclosed by the outer shell of ultrasound probe and the remain solid will be what I want. However, several issues are encountered. I have no idea to obtain the surface object of the cavity. The cavity surface comprises of different parts from 3 objects "Surface-Imported1", "Surface-Imported3", and "Imported1".  Do I need to combine the 3 objects first in order to get a single surface? How to achieve it? I probably can perform a surface cut if I have the cavity surface.

Here is some pictures for better illustrations. Parts highlighted in red is what I want to get rid of and highlighted in green should be remained.

Cross-section view of front plane

Cross-section view of side (right) plane

Surface-Imported1

Surface-Imported3

Imported1

The 3D model is attached please feel free to look at it.

Thanks in advance =)