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I would caution against laser cutting polyurethane, it can release hydrogen cyanide gas which is quite toxic.I have had very good results making laser cut molds using MDF. The uniformity of the material, with no grain, helps produce a smoother finish. Also, I found that a preheat of the mold in the oven at 300 degrees helped remove moisture (and the resulting air pockets in the casting) and allowed the metal to flow and fill the mold better.
Unless this is to hold a pickle, you might want to check the spelling on the cover photo. :) Great design and instructable though!