When transporting a kayak, a cockpit cover cuts wind drag and keeps the rain out of the boat.
They're particularly important if you're carrying the kayaks flat rather than on their side.
Trouble is, they tend to be rather expensive.
This design (scroll down), by Bruce & Mark at Lomo is just £15, which is more than half some of other models out there.
It even has a handy clip to stop the cover blowing off the roof if accidentally caught by the wind.