Hi Derek, an example model is attached here.

So note that in addition to the piece which will be the cavity I have also drawn a dividing line there. When you do the boolean difference, select both your part and the line as cutting objects to make the result shown in the right.

I'm not sure exactly where you want it to be cut, but position the line relative to your shape accordingly. If you need to have pieces that stick out past the dividing line then you may want to union back together some pieces after the cut.

- Michael