I fitted a 100 litre holding tank on a shelf and against the aft bulkhead of the engine room.
Having listened to horror stories about blocked outlets I used a 38ml outlet from the bottom to a new 38mm seacock fitted vertically under the tank and starboard side of the calorifier. I have also taken an outlet (closed end) under the toilet shower divide up to deck head in shower cupboard in preparation. I have not fitted a deck fitting for pumpout since I have found only 1 facility from Spain to Turkey. I ran the diverter vavle outlet in the same divider.
You DO need a breather. My 22mm is adequate and fed over the internal molding between galley and aft cabin and down to waterline. This is a problem when draining tank and when heeled.
I now think a 50 litre tank would suffice and mounted against the engine room deckhead.
I purchased the tank and valves from Tek Tanks but there are other cheaper fuel tanks around.
Contact me for further info