I don't remember if FloXpress reports the internal volume of a cavity. Certainly it could, and it's not difficult to use, once you get the hang of it.
Or you could subtract a copy of the part from a non-hollow solid with the same exterior geometry, yielding a new solid with the volume of the original object's interior.
Or... I think there are some plastic- molding oriented commands that can generate or report internal volumes.
Mike Halloran
Pembroke Pines, FL, USA