The 1980 Car and Locomotive Cyc shows a 1 1/4 inch square shaft with a 16" brake wheel.
This scales to a little more than .014 in HO. I found staples that match this dimension with a little filing to make it square. The steel is much stronger than brass in this size.
While this is newer than steam era, I doubt there was much change. The square shaft is on a drop shaft hand brake. Does anyone make a model of a drop shaft hand brake for flat cars? My detailing of some Proto 2000 flats is held up by a lack of this part.
