You may want to try some of the remedies that Drs. Foster and Smith offer on their website at
drsfosterandsmith.com
I haven't tried any of them yet, thank goodness Miss Bo is a lady when it comes to "off-gassing".... but our old orange boy could really let a few that smelled like something had died!!
Drs. Foster and Smith are two vets with a great website and catalog that has hundreds of handy things for cats, dogs and other pets.