My rear hatch is finished (with magnets) and the front hatch is curing with 2 gallons of water in a doubled trashbag pressure setup. Vaclav recommends a bag of wet sand, but I only have clay and rocks, and lots of leaves.

Trust me, there is a hatch and CF/Fiber Glass layup under there. The hatch and boat is protected by clear shelf paper. Hatch is duct taped from outside and reinforced with fiber packing tape so the hatch will stay under the pressure of the water.