Yes, I got that I should stick to 60 X 10 which is what I am now looking for, flat as much as possible provided I can stand up anywhere. With the upper corners narrower, if possible, than the base. Insulated and well heated, no water in, no heat out, toilet with holding tank. GRP problematic for insulation, unless exceptions (also less character honestly), sliding decks to be checked for proper sealing, possibly simple hydraulics, if at all. I am adding as short as possible, provided enough space to work and live comfortably in 3. That is what I got about the limits thanks to your help. I also learned that I will need to learn a huge amount about the water systems as they are a lot more dynamic than I thought. I will come back on that