I made a bowling ball in college for an engineering competition. we had to use concrete for our ball.
I made a mold from high-density polystyrene, using a C and C machine. my mold was 2 hemispheres with 4 tabs for bolting togethers. the ball surface came out smooth as glass.
if this is a school project, you might be able to find someone that would donate some material and time on the C and C machine.
in my case, the material was left overs from the mold we made for the Concrete canoe that we raced. and we had a local company with a brand new 5 axis C and C machine that they were breaking in. our canoe mold and my bowling ball mold were the first things they did on the machine. that way they got use to the new machine w/o risking their paying customers orders.