Are you talking about flat sheet of plastic? In that case it is Lexan (Polycarbonate) and not Plexiglass. Flat Lexan was furnished with early kits. Van's recently upgraded to molded Plexiglass which is impervious to spilled gasoline.
Apropos of nothing I wish that rear window was a hatch into the baggage area like my old mooney...
Maybe one of the smart guys can sort that out and sell a kit?
I'll sell you my original Lexan still in the wrap. Make you a deal you can't refuse. While we're at it, I'll make you an amazing deal on the original nose gear.