After much searching, I've decided that I may as well just use concrete for my hollow-sphere speaker project. By using casting foam to make a mold of a hemispherical wooden bowl ($20 from Ikea) with all the requisite details added, using modelling clay, I can cast four identical concrete hemispheres which can be combined into two nice neat spheres.
The problem, however, is that I don't know much about concrete beyond having once helped put a bit in my backyard. With the right additives and enough mesh, a ~1/2" thick wall should be possible, but I'm not sure exactly how. Anywho, I have the following questions:
1. How do I make it stronger? There are several answers to this questions, including mixing in chopped fiberglass, chemical additives, and various types of plastic and metal mesh. What method would you recommend?
2. How do I make it easier to pour? I'm aware that there are several types of plasticizers and similar chemicals to choose from - which ones work well?
3. How do I improve the aesthetic appeal? If anyone knows any tricks for tinting, mixing, or polishing, I'd be open to hearing them.
4. How do I join the hemispheres together after casting? I'm a bit stumped on this one.