Wes Kisting is building the "Ultimate Expedition Kayak" and giving the details on RoguePaddler.com. This is a detailed description about how used rare-earth magnets to hold his day hatches in place.

This article on Ted Henry's pretty site details his approach to magnetic hatch installation.

Nice approach to securing flush mounted hatch covers that stay water tight. Ocean-kayak has written a nice article on this approach using clear instructions and illustrations.

I am thinking of using this approach on my kayak, but will probably add a little more support structure to the underside of the hatch. This design puts all the downward force in the dead-center of the hatch.

Ross Leidy has thoroughly documented how he creates a super clean flush mount hatch cover using bungee cords inside the hatch compartment.