Tim O'Connor wrote:

After attending Frank Hodina's clinic on making resin masters for
single sheathed box cars, I am much more willing to try to create
something new than I was before. And you don't have to make masters.
You can just make two identical sides from styrene.

Tim, thanks for the words of encouragement!

Frank fabricates hat sections with .005 styrene base (.060 wide?)
with a styrene strip (.040x.060?) for the raised part. (You can
figure out the correct sizes to use if you have a drawing.) He says
a standard horizontal board width was 3 1/4" (or something like
that) that you can use standard Evergreen strips for.

Just for the record:
There are good-resolution photos of GBW 8104 and 6156 (similar single
door cars) in "Green Bay & Western" by Stan Mailer (Hundman
Publishing). The boards scale out to be 5" wide.

The GBW 15000-15098 double door cars seem to have been overlooked by
rail photographers so I don't have as specific informationfor them,
but it is likely that they had the same board width since both the
GBW 8100- and 15000- cars were built by Bettendorf at the same time.