After having tried the following kayak transport aids, I've decided the Spring Creek cradles are the best of the bunch:
The Spring Creek cradles (sold by Castle Craft) appear to be the most durable and well-made of any cradles I've seen. The machined aluminum clamps are thick, stiff, and fastened with substantial bolts. The rubber cradles are extra-long AND flexible AND sticky, so they wrap around the hull and conform to it better than any other product (this is with a Tempest 165--they might not fit my S&G's hard-edged hard-chined hull as well). Probably help to avoid denting also, as the holding pressure is spread out a fair way up the hull sides, not just at/near the bottom.
- Padded 2x4s: simple and cheap but dent hulls; not a secure hold without overly tight strapping.
- Custom homemade wood-and-minicell-foam cradles: perfect for the S&G kayak I built them for but do not fit other hull shapes.
- Thule 835XT Hull-a-Port J cradles: pads wear out quickly at pressure points; plastic parts seem less than durable; weird shape for sea kayaks.
My husband and I normally use a trailer with other pieces (see above list) but we bought Spring Creek cradles for rooftopping on a very long road trip with ferry stretch, 5000 miles between Colorado and British Columbia/Alaska. They held up beautifully with no problems whatsoever.
The one downside is that, like most non-J cradles, it takes a while to adjust them to the right positions and spans when initially setting them up. But that exacting set-up is one of the things that makes them work so well. The fit is very tuneable, unlike with J cradles.
Although we first intended to keep the Spring Creek cradles on his rooftop bar set, I just might take mine off and figure out some way to use them on the trailer while leaving the padded 2x4s in place for other purposes. They are superior to everything I've seen so far, though I have to admit that I've never seen Saris cradles.