What you need:
1. Styrofoam - Pre-cut shapes from the local craft store
2. Plaster of Paris - Home improvement store
3. Paint - Spray paint from the craft store
4. Aquarium line tubing - The green stuff from the pet store
5. Cylinder - A giant vase from the craft store
Step 1: Cut and Glue
Start by cutting the large sphere and small spheres in half. You can use a hot wire cutter or a serrated kitchen knife (That's what I used).
Then glue the two cylinders together, with a low temperature hot glue gun (IMPORTANT - a high temperature hot glue gun will melt the Styrofoam). You can also use school glue or wood glue. Be sure to test the glue first to make sure it won't melt the Styrofoam.
Now glue 1/2 of the large sphere to the center of the disk.
Step 2: Plaster
After each coat of plaster you want to do some sanding. It is best to sand when the plaster is hard but not entirely dry, that way you aren't breathing toxic plaster dust.
The best thing about this process is that if you make a mistake just put some more plaster on and sand it down.
For my prop I used about three coats of plaster.
After you are done let it dry.
Step 3: Paint
I painted the base and top black, and the small spheres metallic silver.
Do a lot of light coats. You can always put more on but if you put on too much it is very difficult to take off. Just go slow and put on a lot of thin, light coats.
Step 4: Drill and Tube
Pick a drill bit that is the same size as the aquarium line tubing. Then drill holes everywhere you want to connect your aquarium line tubing. You don't need to drill them that deep, just deep enough to stick your tubing in.
When you drill your holes you will chip off a little paint but that's ok. Just pick up a paint brush and touch up the holes with some of your latex paint.