Random mold making tip #1 – corrugated plastic (often used on outdoor signage or political signs – is perfect for fast and accurate mold making.  since there are two layers, cut one layer and form the board around your model.  Here I used silicone caulk (the same you can use in a bathroom for tile work) to hold it in place as well as binder clips – another time saver.  In a pinch you can also use duct tape, but only for smaller molds.