I would suggest this grounding point...
... or direct battery ground post connection.
The problem that I have with your Combox chassis ground is that you will be sharing that chassis ground with a high powered amplifier. Although the assumption is that the Combox chassis is fully isolated electrically, nevertheless the assumption is that it is fully isolated electrically
as is in that chassis location. Not to share that location chassis ground with a high power amplifier.
Perhaps is no big deal, but again there are better ground points than this one.