In an ideal model railway world, people would produce replacement parts for popular kits. That would mean I'd have been able to buy some nicely moulded replacement windows for the engine shed. Sadly, despite buying several different makes, none of them fit the holes in the side of the building.
Airfix supply some reasonable mouldings which are intended to fit from the inside. There is a flange around the edge to stop them falling out. If you look inside it's horrid but then when this kit was designed, that was the realm of super detail so no one bothered. I am fitting an interior so this at least had to be removed.
Test fitting showed I was going to have to thin the moulding down by nearly 2mm before it fitted in the hole flush with the inside and slightly inset from the outside. A 6 inch course file did most of the work with finishing off on a sanding block fitted with fine paper. This process makes a surprising amount of mess, but it's worth the effort I think.