I made a sphere a couple of years ago (equal parts Portland cement, sand, peat) using a replacement glass globe from a light fixture. I have a larger globe, maybe a foot diameter, and want to use it as a mold. My question is, can I partially fill it, insert a smallish ball in the center, then finish packing the rest of the mold? Will it be strong enough with a hollow center? The reason I want to do it is so the finished globe won't be so heavy.