Yakima Mako Saddles
The Mako saddle clamps on to a round or square bar. I installed them on a round bar, but there are little plastic tabs that can be removed which make it compatible with a square bar too. Installation is easy, with no tools required. Brackets are first snapped on to the bar, then the saddles are attached with a bolt and wingnut. Due to no necessary tools, it is also easy to make adjustments while on the road.
The Mako are sold in a pair, but two pairs are needed to carry one kayak. One for the front bar, and one for the back. I bought four pairs to carry two kayaks. I tested travel up to 80mph with two kayaks, and they held solid.
There are also some nice extras included in the box. There is a tie-down strap with rubber covering the cam buckle, which protects both your boat and the roof of your car. Also included is a bow or stern tie-down which hooks the end of your boat to the car's tow hitch. And each saddle comes with optional felt padding, to protect the boat. I found that with the included straps, nothing additional was needed to secure a kayak.
One thing to note is that these saddles are not compatible with a locking core. If a lock is needed, or if attaching to a Whispbar, then look at the Yakima Mako Aero saddles instead.