I usually chuck a strop over the top but only do it up firm and use poly to spread the weight of the tank, that coupled with having it packed out really well with poly usually makes sure they dont move at all, but have also stuck poly under and in front and used a strop to pull tanks forward into the front of the trailer.. We are only talking about 2-3ft tanks here so should be very easy to move/transport, I would question the cost benefits of hiring a moving company compared to the replacement cost of second hand tanks of this size.