Hi Robin, the portable toilet I installed is the Dometic 970 series toilet which I mentioned in my first reply. They make two toilets in this range a 2.5 and 5 gallon version. I originally fitted the 2.5 gallon but swapped the base for the 5 gallon version after the first year because I wanted a larger holding tank capacity, otherwise the units are identical. They are available form various chandleries and more commonly from camping/RV/caravan suppliers as well as from Dometic UK directly. It has worked well although I still occasionally have issues with slight odours which is why I am now considering changing to a composting toilet but it will have to be a a home built version as most commercial versions which are designed for larger boats or home use where space is not an issue. They are also hugely expensive.I have a design which I am considering modifying to fit in my boat and which will be much smaller. All the articles I have read about composting toilets extoll the virtues of not only biologically green use but also NO ODOURS. As far as where to fit the outlet for your boat, as long as the have a point where you are not going to drill through the ballast, anywhere low down would be fine but without knowing the particular layout of your boat, I cannot suggest anything more. On my boat which had the fitting already fitted, it appears to pass through the hull just at the edge of the encapsulated ballast in the heads area immediately inside the heads doorway and on the outboard side of the heads doorway. I would suggest you fit a Marelon/Forspar valve as then you will not need to worry about corrosion issues. Just be sure to use silicon sealant, NOT sikaflex or similar as the normal sealing mastics can affect the plastic of the fitting causing failure.