I have an assembly that is a combination of parts that creates a cavity. Gas flows through the cavity and exits out the top end of the assembly through a circular array of burner ports. I want to know the volume of this cavity from the inlet to the outlet so I can run a fluent model. I tried the plane and intersect tool method but could not get the cavity to fill up.

I used the method in the below video to join the parts, create two planes and run the intersect tool. Any help would be appreciated. I have attached my part and assembly files.

SolidWorks Tutorial - Fill an Assembly Cavity to Determine its Volume - YouTube