That is a superb job; thanks for sharing it including the historical material and plans. I think that for the sub-scale interiors of most Lionel, MTH and K-Line cars, scratch-building interior details like you have done is the only way to go.
I am not sure that anyone has produced anything in the way of 3D printed details for 3rail RPO cars but Delta Models makes kits and components in cast resin. Problem is that these are intended for cars with scale-size interiors - more like Golden Gate Depot or brass 2rail models than the main manufacturers’ products. An example is the Delta RPO overhead bin (like the ones shown in your third photo above):
This is a nice replica of the real thing but would have to be cut down with some difficulty to fit in a standard Lionel etc. RPO. I see how you have tackled this part in your car and bearing in mind what can actually be seen through the windows I think it works great.
Delta make other parts and indeed whole RPO interior sets. I’m only showing a few here for comparison purposes:
