Jim, I use the bungee designed to hold the Mirage Drive fins "up against the hull", to hold the seat back down on top of the seat bottom.

With the seat folded down, feed the bungee through the opening in the mesh on the R/H curved part of the seat and back frame and then use the plastic hook on the bungee to grab the strap that tightens the seat-back mesh.
This is a foolproof method that will not come loose.
I can take a photo and post it, if my description is not clear/fuzzy.

How do you all keep the back of your Vantage seat from flipping up when trailering on the highway? When new, I could fold the seatback down and it stayed down at highway speeds. Now, my 2-month old PA14 wants to travel with the seatback up. Is there a tension adjustment in the hinge somewhere? If not, I can attach a Velcro strap or even a shoelace to tie it down, but am looking for a more elegant solution that won't be lost or left behind somewhere. Attaching paired rare-earth magnets inside the contact surfaces of the back and bottom frame tubes comes to mind. I doubt that I am alone here - what workarounds have you all come up with?

TWS- that's an interesting perspective! Makes perfect sense. In my case, though, using a cheapie HF trailer, I don't think that I would be able to keep the rudder and skeg in the down position on the trailer if loaded "backwards". Even if I could, I would worry about wear-and-tear on the steering lines if "road wind" tried to deflect the rudder sideways while in transit.

Why do you have to trailer with the rudder down? Not sure what that would accomplish. If you are wanting to remove the tension on the lines, then let off a little on the tension but it will stay in the up position with less tension. Give it a try.

Jim I'd just toss those plano boxes in you front stowage compartment or your vehicle during transport and use the straps provided-thats what I always have done-simple. With as much expensive gear I have in those boxes I'd never travel them out in the open like that.