On the trailer I have, there are two and they are bolted down to the wood deck. Just build something to hold them stead and they travel just fine upright. My only concern sometimes is the open vent facing the 70mph wind, so I cover the vent.
For traveling, I just knock down the insides, take the racks, place them all on the lowest rung and the weight travels fine.
Oh, and don't forget the grease tray. It WILL bounce out if you leave it there. and after cooking in a comp and you didn't empty the tray and left it there, all the bouncing MIGHT make it bounce out and fling grease all over the place. Not that it happened to me, nope, just heard that somewhere. That's my stupid story and I'm sticking too it, no evidence anywhere.