I just replaced mine yesterday. The must be plexiglass. The reason is the window is curve side to side and up and down.
Scamp sells the
windows and gaskets. Get the single piece gasket. It is possible to source the plastic local. Be prepared to have to sand and modify the
fiberglass openings to accommodate new windows.
Also be prepared to pay to have installed.
Hope that helps.
I spent 150 for installation and 280 for materials