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


Jeff Southall
Current boats
Nacra 5.8 1703 Animal Scanning Services
Nacra 5.8 1667 Ram Raider
Nacra 18 Square
Arrow 1576