Based on all your inputs I was able to put it back in. The trick is that pushing hard DOWN on the frame does not work. I pushed it SIDEWAYS with a piece of wood, pushing the lip away from the edge where it has to 'clip' under. Then a light push downward and some wiggling on the rubber did the job. Had some WD40 helping too. Note that the picture below is an old one of a test fit, just to show what I mean with 'sideways'.
