I am trying to find the volume of the voids in my hollow part.  So far I have been creating 2 reference planes on each flat side of my part.  Then I use the intersect function and select both planes.  This does not work - what am I missing? 

I should mention that some tutorials say to also select the part body when using the intersect function but for SW2016 the program explicitly does not let me select the part when using the intersect function.