Sphere as the main electronic housing


Hi Guys, I'd like to build my ROV with a sphere as the housing for the electronics to try and achieve far greater depths, I'm thinking about 6mm thick Polycarbonate or acrylic hemishperes but can't seem to find anything like it on the internet, I've found 3mm ones on ebay but thats all. Anyone have any ideas where I might find say a 250mm diameter hemisphere as thick as possible?