If the cut consists of >>1 feature you could also try a cutout in assemblymode.
Assemble your model (with the cut in it). Create a new part and assemble with your model. The new part should have solid geometry filling the cut completely. Then you do a cut-out to the new part using your part as reference. The result in the new part is the cavity, and you can do all the analysis you want.
We use this method to visualize cavities in complex molded products.