This should work but I haven't tried it with your model:
1) Add sketches & extrusions as needed to your model so that the bottom opening is all on one plane. It doesn't matter it the material you add projects a little into the interior. Make sure the result is a single body.
2) Make a sketch on the bottom surface of the shell and draw a rectangle a little bigger than your model.
2) Extrude the sketch as "new body"and extrude it right through your car model.
3) Use Modify/Combine with the "cut" option and use your car model as the tool body. Make sure you check the box to keep the tool body, otherwise your car model will disappear. The new extrusion will be cut into two pieces.
4) Right click on the body you don't need and select "remove"
3) Use Modify/Combine to join the remaining two bodies into one.
Alternately, skip step (1) and add a step (2a) where you sketch on the side of the new body and cut it off flush with the bottom of your car model. The important thing is that you want your car model to cut this body into two separate pieces in step 3.
