Mark out your window frame on some abs sheet. Score around the lines with a needle. Press out rectangular shapes. Place grid on sheet of perspex. Trim to size. Use epoxy not CA to bond two sheets together (unless you want crazed glass effect).
You may want to look at Grandt Line’s windows. There’s nothing there as large as what you’d need (I’m assuming you want to do the shop windows). There are 3 x 6 pane windows there that match the windows used in many of the EBT’s depots.