I made a setup with a waterproof bag in the center hatch.viewtopic.php?f=75&t=32772&start=30
As mentioned here I have another reason to do this. It is now tested and is working just fine.
I can access things under the hatch "outside" the bag even when in big waves without the any risk to fill the kayak with water from the sea. I can use a pump with very limited risk of getting water back in. If I want to access stuff inside the kayak, I can easily open the bag and reach for things without rain coming in.
In good weather, when I dont need the function of the bag, I can just roll it up in open mode and put it like a collar inside the hatch. To dismount the bag, I just replace it with one normal sealing between hatch and hull. Nothing is glued to the kayak, no impacts on the kayak.
For me this is working so good that the bag function is always used. Things needed when under way is of course limited to a minimum. Any amount of water coming in during an opening of the hatch stays above the closed bag.
If any bigger amounts of water get in, it is just to lift the bag (in closed mode) to empty it from water.
Thomas in Sweden