FYI, I have found the Lexan is cuttable (using a skill saw) but plexiglass will crack when trying to cut it.
Also, the first mistake I made was using the plexiglass that was in the window when I got it as a pattern. This piece was not well done and did not fit well. I should have checked this.
Also, though the window is oval, my window is not symetric. I guess who ever did it just eyed it and did the best. It is not realy noticable, until you cut the Lexan and find out it still does not fit.
I ended up having my wife hold the Lexan up against the trailer while I traced it from the inside.
Hope this save you some money and cutting and aggravation.
Also let me add that the rear window is also your fire escape. However you secure it, make sure that a good kick from the weakest in your trailer can dislodge the glass. It will be hard to break this from the outside given the curve and thickness of the plexi.