Your welcome. I should add that I don't know FOR SURE that your hole size is 20x20 - just that it's the most common. But, either way, it's easy enough to measure then find one that fits. Most likely, it's still going to be a commonly available size.
I have "heard" that some people have removed the plexi by gently heating the adhesive caulk that holds it in and then replacing the plexi. Note that I'm just regurgitating some info there - I have no actual experience doing this. But, it might be worthwhile to check into that.