When you were looking for hull bits, did you find a fabricator who could and would do non-standard elliptic heads? I'm fiddlig with the old Diving Saucer shape, which was 1.5 meters by 2 meters, give or take an inch, and was 3/4" thick. I like the space and payload available with that shape. A 2:1 head gives you, what, four feet in height? I don't remember the thickness of your hull. What does the shell weigh and what depth are you figuring for test and operating?