Just plug it and dump some CLR in there. Will be like brand new.

Traps are made with both slip joints and solid glue in style. Code prohibits installing a slip joint type trap in a concealed location, therefore when installing a skirted tub we are required to use the glue in type unless there is a 12x12" service access panel either through an adjacent wall or up from underneath. In this instance the proper solution would be to cut out the existing trap, then install a new glue in trap. Traps are relatively cheap, and you should be able to purchase a new trap and a coupling for approximately the same price as a fernco coupling and you will have the job done correctly.

Thank you for the reply! But I'm still stuck. Are you saying that there is a screw on the side? I'm not finding one. If you mean removing the cover to find the screw, I should clarify that I am unable to remove the cover. Should it definitely turn? I've tried turning it as hard as I can with my hands, and I don't want to wreck anything. This has only been in place for a year and a half so I would not expect anything to be really stuck.

Well i don't have a plumber on retainer. I'm asking for a decent quality universal fit as opposed to the $9 chrome one I can get from the only real plumbing supply place within 50 miles (which is where the plumbers get their stuff). Went to Pacific Sales --they have no drain stoppers on display and told me they couldn't help me, just referred me to the plumbing supply place with the $9 ones.