It really is not too difficult. Just may be time-locked into place.
First work the inside lip of the gasket, loosening it all around. Pull it, fold it over, to break things loose.
I think I then went for the bottom corner, pushing from inside with one hand, pulling with the other from outside.
Things should start to give away and it is just a matter of folding back the gasket, pushing, folding, etc.
Larry
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