Not to plug Peggie Hall's book(let) on marine sanitation devices but her book does explain all of the nuances that can make a boat smell sickly. I hate a smelly boat worse then the next person so I bought the book from SBO (sailboatowners.com). I have found that her tips and insights help immensely.
On our own boat whose head is not often used we:
-added a second vent hose from the tank (cross ventilation if you will)
-use only KO from Raritan as the treatment for the holding tank and no harsh chemicals like bleach etc.
-cleaned and painted the bilge that I could reach
-never overfill the tank as the air vent then becomes the likely route for the excess (don't ask how I know this) and may drip in the bilge from any fitting that has been compromised by overfilling or vacuum pressure created from a pump out.
One of the main points in her book(let) is that anaerobic bacteria (no oxygen) will create a stinky sulphurous smell but the bacteria in the KO product with enough oxygen will hardly stink at all as the waste is broken down. Pumping out and flushing the holding tank frequently will also help minimize odors as will cleaning out the bilge the old fashioned way (then paint it). Another major point not to be overlooked is to keep your holding tank vent lines clear of any blockages as any blockage will likely cause the system to fail to be pumped out completely and create a vacuum in a system not designed to be vacuumed out. Clean vent(s) to the tank are key to achieving a clean pump out of the tank.
Covering the top of the head (John, WC, whatever) even with a vented cover is likely to cause unintended consequences as the boat heels on one tack or another. Just because you don't see 'stuff' backing up into the bowl does not mean that you wont smell it. Getting a pressure seal on top of the head that will hold back the 'liquid' is unlikely. Having a properly plumbed head with the right treatment and maintenance in the first place will save you from having to take extraordinary measures.
Peggie Hall's (not Hill) book: Get Rid of Boat Odors
also contains diagrams of the most popular heads sold on the market.
How is your summer going? Care for a sail up at Nyack before the white stuff comes and/or will I see you in November when Bene505 moves his boat from MTK to Glen Cove again?
Hope you are well.