I have an idea for sealing the hatch this which may work:
First it involves fitting two short pieces of 1/4″ stainless bar which are screwed to the deck, stern side of the hatch using u shaped clamp fittings.
Lengths of cargo webbing are slipped round the bars.
The webbing then passes over the hatch and halfway down the tank. The straps could be clamped in place on the upright surface using what used to be referred to as “bonnet catches”. These are made of rubber and half a kind of locking mechanism that stop them coming unlatched.