Step 1: Sketch shape you wish to mold, keeping in mind undercuts in molding process
For my bicycle music system, I chose a shape that was relatively easy to mold with a left / right mold.
Step 2: Create a rough version of the shape, in foam.
In the series of photos you see that I stacked the templates (which are made by 'tracing' a master template with a router) with threaded rod. Then I shot expanding foam into the form (inside a garbage bag.)
Then I used a hot knife to slice away the excess foam. The masonite templates stop the knife from cutting too far.