The mold can be made out of foam, mdf or other stable and shapeable materials. The denser the material, the more it will hold up over multiple pressings, but the harder it will be to shape.
Let's get started:
Set the blade of the utility knife just above the depth of the side profile line and make a series of cuts across the foam, about 1" apart.
You can see the cuts I made on the tail of the deck in this photo. Make sure you do not cut below the profile line. Using a saw cut above the profile line as shown. If you don’t have a saw, the utility knife blade will work as well, turned on it’s edge.
Remove the excess pieces of foam.
A little messy looking but don’t worry, you will be smoothing this out with the sanding block.