how to find resonant frequencies of metallic housing using HFSS?

I am designing housings(metal enclosures/box) which will house circuits operating at C-band.
Another housing for X-Band.

I want to ensure that box does not resonate at these operating frequencies.

box dimensions : LXBXH=190 X 40 X 15 cu. mm for C-Band,with 50mil thick substrate.
=100X 35 X15 cu.mm for X-Band,with 25 mil thick substrate

Want to do this in HFSS.
Whether eigen mode solution will give correct idea of resonant frequencies,how to decide
whether box will have no resonance at operating frequencies after seeing the eigen mode solution.

Experts please help.