I finished the design for dual cavity mount. The assembly looks fine, all parts fit together. I'll make sure that the mount can be screwed down to the current seismic stack before I submit the drawing.

After finding the optimum support points using COMSOL, I redid my cavity support design. The picture below shows the assembly of a metal base, peek pieces for support points, and copper shield. The picture shows only half of the mount.

Personal note: 

The current design, the beam height is 1.32 inch above the top seismic stack, ~5.5 inch measured from the table.

With the new cavity mount design, the beam height will be 1.5 inch above the top seismic stack.