I am not sure if this will work with all window arms but for the years like our 1979, it should work fine. We took the solar screen material that we did the screening with (which is nylon), cut strips 1.5" X 1" over the height of the slots. Took each strip and folded it in half and using hot glue, attached one strip to each side of each slot and overlapped each folded edge by about 1/2". This allows the flat window arm to slide through without any gaps for the bugs to enter.
Photo of the front of the screen slot:
Photo of the back side of the screen:
Close-up of the screen slot after install: