You are quite correct to be skeptical of the ability of the system shown to keep the hatch sealed tight. The problem is that the center is held tight but not the outer edges. Pygmy has solved the problem of applying pressure to the edges as well as the center of the hatch. You could very easily adapt their system to use bungees instead of straps.

But even so, bungees will give. Say when a big wave washes over or when you are surf landing. The water catches under the edge and even a slight raise of the edge dooms you. Pygmy's hatch straps don't give. I have surf played many times with no water getting in