Drain the fuel out.
Use some type of centerstand or other means of keeping the bike straight upright.
tie off to the load strap points on both sides of the box, level with where the straps hold the bike.
Nuthin' to it.
I haul my Harleys, dirtbike and used to haul a zx600 race bike. As long as they are properly tied down I have never had a problem with fuel leaking and my trailer smelling like gas. The only problem I have had is the plastic gas can expanding from the heat in the tailer. I always vent the can. Then you could get alittle fuel smell.