So I cut some 1mm thick plasticard panels to fit the gaps and then marked out the window holes by drawing around the window frames that I got from https://www.jandmhughesogauge.com
Then trial fitted…
…before scoring around the outside of the windows between the lines the hole so that the frame will overlap both the window hole and the glazing. Then cut an x using a Dremel so the remaining plastic could be snapped off.
After some more filing glued in permanently using the polystyrene cement leaving slightly proud along the bottom edge due to a slight wasting of the body below the windows. Filler will hopefully then fill the gap to give a smooth line up the side of the body.
Apply some filler
This is after about 4 cycles of sanding and filling.
You may have noticed I’ve also filled the holes in the roof with plasticard, squared off the tops of the end doors and opened up the small windows. It doesn’t matter that this window is still in the original thick body as it is the toilet and will be opaque white so you won’t see that thickness.
Maybe rain guttering and other details.