I have just spent an afternoon with a slide hammer trying to get this thing to come off the filler pipe and all I have managed to do is get it to move about 1/4". I can tell it is a rubber type fitting as it moves with pressure but won't actually come up. The overflow tube that goes under it is angled which seems like about the worst decision ever (maybe it turns?). Does that mean the whole thing has to turn while coming out to get the pipe out? Am I missing something here? It's the big NSS labeled part in this diagram.

 

So thinking about it I went back and started wiggling it back and forth prying a bit with a curved seal puller and about an hour later it wiggled free. If you need to do it in the future I would say skip the slide hammer or any sudden force and put instead pressure back and forth trying not to damage the sheet metal.

 

As soon as you get it up a little, start squirting some lubricant WD40 or similar underneath. Right at the outside seem.
Absolutely flood it, then let is sit and soak in. I ruined one cap trying the twisting. Method
Luckily I have 4 extra caps. ( now 3.)