I live in Maryland so I can't speak to the extreme cold but I did have a
roto seakayak that I car-topped a lot. Twisting the straps seems to cut
down on the vibrations. In order to keep pressure from the straps
deforming the hulls I cut a piece of ethafoam and make a temporary wall
which I wedge inside the hull at the location of the straps. I store
the boats with the temporary walls and just take them out when I am
paddling. The saddles will protect the boat better than the straight
racks will but it still won't hurt to put walls under the straps unless
the straps happen to be where your existing bulkheads are located. The
warm Florida sun may be more of a problem than the cold. The Plastic
can get really soft & flexible when it is hot.

Jack Seppelt wrote:

My wife and I are about to leave eastern Iowa for a few weeks in the warm
Florida sun. We are considering hauling our kayaks and I am concerned about
their exposure to the current outside temps for the first few days. It is
currently reaching -5 F to -10 F at night with daytime highs are in the
single digits. The kayaks are Current Design rotomolded plastic and I haul
them on top of the car. One on foam pads and the other on Yakama Mako
Saddle and Hully Roller. I am worried about the pressure from the straps
and the vibrating on the cold plastic. I have no idea how brittle it will
get at these temps.

Any experience or opinions will be appreciated. Thank you.

Jack