Well, I have gone down the resin casting route before. The issue was making the casting thin enough to house many of the LED top mounted units people have been using. The beauty of the 0.8 mm alloy top is that maximum space can be achieved internally, where the Perspex lens block locates in the top section, this is thickened with plastic card to give the appearance it is approximately 2 mm thick. The alloy also gives more options for different stalk mounting options.