If you don't want to chew through multiple Railsmith cars at $64.00 each, the ancient Con-cor dome that they sold with many of their sets 50 years ago, which is based on the same car as Lowell's, has windows that scale (according to my trusty cheap Chinese calipers and my calculator app) to 71.5" x 29.75". And that is measured from a carbody that I use for testing paint. If it were stripped, it might well be 72x30.
Shown on this page as "mid train dome"-
https://www.spookshow.net/passenger/ccsmooth.htmlNot knowing what you are trying to model, so this may or may not apply. But when I am faced with a kitbashing challenge, I check the product listings at Union Station products to see if he makes sides for the prototype I want to model- can save a lot of cutting and pasting. And a set of sides is often cheaper than buying several donor cars for the parts I need.