You have to make your own...
...however, please keep in mind I am referencing a Hunter 31. Not sure of your boat model. If we are in the same "boat", your freshwater tank is under the port quarter berth and comes with no access. The top panel is attached to the tank with with a slew of screws and possibly some sort of caulking and/or adhesive. Way too big a job to take it off. Therefore, I drilled three holes in the lid and installed waterproof access ports in each. There is an internal baffle in the tank that runs diagonally from the forward starboard corner of the tank to the aft port corner. From these ports, it is a reasonably simple task to reach in with a toilet brush and scrub the tank.