Zipper Flapper Under RV For Access To Slide Motor

I have a zipper flapper that gives me access to the slide motor under the RV. The zipper flapper is broke and needs replacing. Mine is 18" x 24" long. All I can find is a 18' x 18" zipper flapper. Mine is on a 2006 Carriage Compass, has anyone that has one ever had to replace it. If so, what size was yours and how did you go about installing it. It looks like the outer part has to be glued to the other belly fabric .