ozium, neutralizes alot of odors. or febreze
and since it is just the bottom,,I would consider soaking it in a light cleaner(nothing harsh) with hot water,seat foam an all,and hang it up to dry,,maybe use a blow dyer or heat gun and dry it slowly,,I say this because the smell is probably in the foam too,,also its not hard to remove the cover on the seat bottom its self,,I did my own seats when recovered,,the bottom was easy,, it has hog rings that secure the material over the foam an bottom,,its easy to stretch back on an secure the hog rings,,use a heat gun to keep it pliable and reuse the hog rings,,just take your time when removing