I used this method to create an air duct for the oil cooler on a friend's Porsche GT5R racecar. This is a very quick and easy way to mold a hollow component or a part that has severe undercuts that would not alllow it to be molded in a single piece female mold.
The first step is to cut a pattern from blue insulation foam and wrap it with clear packing tape.
Next, cover the taped pattern with a mold release wax.
Now cover the pattern with woven fiberglass cloth and wet out the resin. Make sure to leave a section uncovered- this is where the foam will be melted.
After the resin has cured, melt out the foam by pouring acetone on the uncovered section (note that in drawing #4 it says kerosene- this is incorrect). Make sure you have a decent sized tray or bucket under your part- this will get messy! As the foam melts you can reach into your hollow part and pull out the packing tape and you are left with a hollow molding.