Did you consider buying a diner interior for the ATSF, diner #600 from NLTW? If the frame you have uses the channels in the shell's extrusion, you can trim the edges off of the 600 diner interior and it will fit within the shell.
If the frame you have doesn't use the shell's channels in the extrusion, buy a frame and interior from NLTW for any 15" diner that comes with an interior and figures.