Rather than lock this away in my build thread, thought it made sense to pop it in here to make it easier to find if anyone is searching for this part.
www.printables.com/model/258068-austin-princess-interior-light-cover
The before is my literally crumbling original lens. The after is a raw print, no sanding or finishing. This is a direct replacement part and, because it's a thing you can 3d print to demand, you can make them any colour you can get the materials you use in. As far as I'm aware, this is the only replacement part out there at the time of writing and this is a part that's been unavailable for quite a long time. Best of all, the download is FREE, so as long as you have access to a 3D printer this part shouldn't cost you a great deal to create.