i have a 5 wide window unit. casement style. 2 panels open, 3 are fixed. 1975 vintage double pane glass. the operating panels have screens that can be removed. we take them out in winter to make appear more uniform. the screens fit into a nice recessed lip and are secured with simple push/pull pins. all 5 windows have the same lip feature. i wonder if i installed plexi panels in the openings and caulked them if it would increase the thermal efficiency of the window? window seems to be in good shape but i do feel a thermal draft. it is 10ft wide so it has a lot of surface area. i could leave the panels in place on 3 windows. i know plexi is not cheap. the panels would be 15"x48". or maybe use single strength glass?