G'day John I modify my own trailers and had the same problem with my mast stand as it had to get over my Landcruiser. To stop the sway I welded some heavy angle under the trailer beneath the mast stand sticking out about 9 inches either side, then welded light flat from the angle up to about two thirds of the mast stand, that makes a rigid triangle and stops the wobble without going to heavy steel and it looks okay without too much windage.
regards