To blow off the hatch off, the wind would have to somehow either:

1) get under the hatch enough to lift it up, which would be very hard, since the hatches are flush

-or-

b) be blowing so hard that the Bernoulli effect would cause enough decrease in air pressure above the hatch so as to have the lid be popped off by the pressure under it. That would have to be a lot of wind to be able to overcome the bungees. Let's assume a 10" round, flat hatch of a kayak on a car moving at 70mph , laminar airflow, 4 bungee attachment points providing 10# each, one atmosphere ambient pressure, and a dog with his head out the passenger's window. What would the headwind have to be to overcome the bungees, and would the dog still have any fur left on his face?

hmmm...

Dean