I figured I'd start a new thread since the previous one is nearly two months old.
I found an N scale X roof for the ExactRail Gunderson boxcar. The Athearn SEICO 50' boxcar has a removable X roof that is a perfect fit. As it turns out, Chris Clune designed both models, and the component arrangement is identical (separate roof, floor/body one piece, separate underframe).
Athearn is out of the undecorated unassembled model. But these cars are easy to find in the shops and online. If you buy one with an aluminum roof, it can be swapped in on the ExactRail model as is.