I was faced with the same dilemma when replacing a 45 gallon holding tank into the bilges. (Not a BCC!) As you can imagine, this is some weight when full of waste.
I did a couple of things; I created a pump dainer for cleaning/servicing, and then fitted the tank into the bilge.
Then I created a couple of stringers, located the tank on the stringers, and filled all voids with expanding foam. Job done, no odor, and no sound!
To remove the tank is easy, just cut through the foam.