Here I would like to talk about my experience with cradles for kayaks.
I soon learned that kayaks can not be transported by just "chucking" them on top of a car and tie them down.
I was wise enough to never do the mistake of just using bungee/ocky straps for securing them (don't laugh, people really use that) and, in the rear vision mirror, see my prized possession tumble down the freeway .
I used proper straps to secure the boat onto the crossbars.
The previous owner of my first kayak told me that she never bothered with those "fancy" cradles that some people use, "the kayak will be fine".
And it was, for about a month.
Then one day I noticed a nice crack just where the kayak met the roof rack.
I must have been a bit heavy handed when securing it, worried it might fall off the roof.
That incident prompted me to learn how to repair glass boats and that after all they have to be handled with care.
While a decent sea kayak can take the force of the surf, a concentrated load in one spot (like a steel bar of a roof rack) can crack the hull.
Plastic kayaks suffer a different type of damage: deformation and oil canning.
I have now tried 3 types of cradles for my kayaks and not all have been successful.
There are several manufacturers of cradles but not all are equal.
Thule Hullavator: a little bit bulky for my style
My preferred one is where the cradle swivels and contours to the shape of the kayak. Some cradles are contoured but rigidly shaped; they might fit just one hull shape but not all. Stress raiser might appear on hulls that are transported on cradles that don't "hug" the kayak evenly.
high-tech layup deforming in tropical heat
Living in a subtropical place my summers get pretty hot.
High tech composite boats often have layups of Kevlar where epoxy is used as laminating resin.
Epoxy's melting point is lower than polyester and in full sun it can become a bit soft.
In one instance I had one very high tech kayak dimple and show stress marks despite transporting it on recommended "J" cradles.
Thule "J" cradles
My best solution for that kayak was to transport it belly up on custom made closed cell foam cradles cut to the exact shape of the deck.
DIY closed cell foam cradles
Since the deck was made of vinylester resin it was more heat resistant.
If your vehicle does not allow for conventional roof racks a foam block can be your only solution, in some cases.
used with permission from British V8 (link here)
My current kayaks don't suffer from heat distortion and I can use standard cradles with hull side down.
Mockpool (SeaBird Designs Northsea) and Sialuk (VCP Nodkapp LV)
A while ago I came across the cradles below.
I am unsure if they would be suitable for kayaks that have light hulls or a plastic one.These pads seems a bit on the small side however they do swivel.
The owner used them to transport a hard chined kayak where the ridge would be strong enough to support his kayak.
Transporting kayaks in the summer heat poses problems.
Before you permanently damage your prized possession on a long hot drive test the hull of your kayak and make sure your cradles offer enough support.