When i put my 36" on first, I put the mower against the front of the trailer and run a strap from the left side vertical "railing", around the back of the mower and over to the right vertical "railing" piece on the side of the trailer.
I also ratchet a 2x10 to the front of the trailer so that the mower rests against it instead of metal on metal.
Please do not just put your mower on and use the brake, too many "professionals do that and their mowers move all over the place". You also can wear down the brakes when you leave the brakes on.
There is an Old Blue Chair for each and every one of us, we just have to find it.
2006 GMC Sierra 2500hd crew cab
BOSS 8'2" Power V
Hustler X-one 60"
Toro Proline 36" wb w/t-bar controls
JS46 22" walk-behind (wouldn't recommend)
Shindaiwa T254, Echo trimmers, chainsaws, pole hedge trimmer, pole saw, hedge trimmer
Shindaiwa 633 and Husqvarna 130BT blower