Assuming the trailer has an upright structure to mount a winch, I would weld the winch to a 1/4 in to 3/8 in. plate big enough to span several supports. Most supports I have seen are 1/4 in. angle iron or channel so I would lean towards this thickness. Next I would bolt it to another plate on the back side of the structures. When you have your own trailer, unbolt it and weld it to the new trailer or just bolt it. Make sure the bolts are rated for more than the pulling capacity of the winch. Industrial bolts have three lines on the bolt head.