If you want to take on a real challenge, you could cut out the window area between the beltrail and letterboard, and replace the "wide diner windows" with "twin windows" from an MTL parlor car. The window spacing is almost identical- so a 6 window section could replace the 6 windows from the diner, eliminating the need to do one window at a time. Double check that the space between the belt and letterboard are the same on both cars (if the spacing on the parlor is greater, that would be ok, too- you would just need to file down the resulting part a bit more.
I won't pretend that this isn't a royal pain in the a**, but with care you can get pretty good results. It will cost you a perfectly good parlor car. However, my experience has been that parlors are the easiest cars to come across at reasonable prices. Also, if you ask Joe or customer service nicely, they will provide a car body if they have one for a very reasonable price.
I don't know if they are still made, but there were window replacement kits marketed once upon a time in HO for doing exactly this sort of thing.
