Tarpaper Roofing

What should I use to form planets with concrete for my Eagle Project Solar System Model?

I am doing an Eagle Project, making a solar system model out of concrete, and I need to make hemispheres for the planets, which will then be put in the main concrete disk for the model. I have been thinking of what to use to form the planets: bowls, tablespoons, etc. but I don't know of something that will work. Bowls have flat tops, and I need dome shapes. The smallest planet would be about 1.5 inches in diameter, 0.75 inches in height, and the largest one, the Sun, would be 12 inches in diameter, and 6 inches in height. Again, I just can't figure out what to use as molds for the hemispheres of the planets and the Sun. Thanks!!
(A hemisphere is one half of a sphere--a ball cut in half--for anyone who doesn't know)

Use balls rather than bowls. Your smaller half sphere can be made freeform and your larger ones over baseballs to bowling balls to basketballs. You can immobilize your ball by burying it halfway it in a bucket of sand. Cut out a circle diameter of the ball out of tarpaper (roofing felt); placed this stencil over the ball to form your hemisphere/hump mold. Built up a layer of concrete, following the directions in the link below for hollow spheres. You just might want to put your balls in a plastic bag to protect the surface of the ball and for easy removal.

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