There are broadly two types of holding tank systems. one is a fully pumped system where the tank is located outside the toilet compartment, usually under a bunk and the toilet outlet is sent to the tank via a Y valve which allows you to either pump direct overboard or into the tank. The waste is removed from the tank either by its own pump through a seacock or through a deck fitting. The second and now more common is a gravity tank such as Yngmar describes. This is much easy and simpler to fit, although it usually requires a custom made tank to fit the space available. On some aft cockpit Bavarias you can also fit a tank in a cockpit locker aft of the toilet compartment but this is not an option on your boat. It may be possible to fit a gravity tank in the engine compartment as heywoodp describes if you can run the 38mm pipes to it from the toilet and back to the seacock. You may find you have less space restriction this way and can use a much cheaper standard moulded tank. The downside of this location is that your deck pumpout and vent will probably have to be in the cockpit rather than on the side deck and hull topside as is normal with tanks located in the toilet compartment.
Avoid metal tanks. Plastic is far superior, either rotary moulded as many standard tanks are or welded fabricated tanks for a custom type. The expert on tank design and manufacture in the UK is
www.tek-tanks.com who sell either a range of fixed size tanks or will manufacture tanks for your specific requirements. There is excellent information on the site about holding tank systems and tank design. You may well find they have already made a tank for your model boat.
Google "holding tanks for boats" and you will a wide range of information and alternate suppliers.
