If you had a design that wasn't open enough to put wood in on the top could you make the two hemispheres hinged? I saw a picture once in a welding magazine from a place in China that made hemispheres and balls. They made everything from tiny little ones about 1/2" in diameter all the way up past 72". They made them out of steel, stainless, and brass with your choice of finishes. The only reason I remember it was that they took a picture of their warehouse full of "balls". 1000s of them every size you could imagine. I wonder if it would be worth pricing them from China if you do enough of them.