As I explained before, it is almost impossible to mark out on the wall. So I bought some grid marked (mm grid) and used a long rule to cut out a perfectly square and measured piece of paper to stick on the wall to show where the hatch will go. It looks a little out when compared to floor and walls and ceiling but at the time the boat was floating and the door steps were all perfectly level. So if the door steps are level then making the paper level ensures that the hatch will match (sorry) the doors when floating. I can then align the ceilings and walls to make things look square.
The afternoon was spent draining the muck out of the battery room. The downside was that the flood occurred all over the really really heavy nicad batteries. Nice combination of very very heavy batteries (3 times that of typical car battery) covered in diesel water, silicon grease and battery acid. They had to go.
So a hour or so of carrying 19 batteries out onto the rear deck and then covered then in ply to keep them out of the sun. Then mopped up the flood and left it to dry.
Final job was to pack up all the tools and the boat is back to clean and ready for the next round of fun.