I converted the (free) porta potty the PO left on the boat to a non urine diverting compost toilet by enlarging the lower casing with a dremel tool and ditching the trap door thing. Since I usually pee into a "plastic something" anyway (one of those tall cylindrical UTZ pretzel mix containers is my favorite and is reusable with a tight seal) it works great. When the GF stays for a week she does all her business in it and uses more (free) sawdust and still no smell. The intact upper water
system helps clean the bowl (pour cup of sawdust down hole first) and dampens the sawdust. Use a disposable chop stick to stir occasionally if you must.
The only problem with this simple, free system is when you don't separate the urine it is harder to make good litter critters, also I recommend two chop sticks for this. On the plus side you can get rid of the cat.
Watch Saturday Night Live Online - Litter Critters - Zimbio