Couple of other thoughts that might help.
First, if you can't get the seat to fold down, load the boat stern forward. That way the seat blows down vs stands up to catch wind. You'll be breaking the ancient superstition of always hauling bow forward but at 60 mph, it's not hard to damage things that flap around.
Second, if you watch that video linked by Tacklesmith, you'll see the straps flapping around. Put a half twist in the strap before tightening to stabilize it.
Third, Take a look at this video and add similar straps to your fenders. It will save you from fishing around the bumper (and save your paint) on each trip. Once the straps are in, it's easy to make a reusable 2 legged line ( to run from the boat to the new fender straps) with Carabineer Clips on each end so that your boat fits in the exact right spot every time. Saves time too.
https://www.youtube.com/watch?v=jtOSM1ezHeo
hth
Good Luck