I've successfully shaped acrylic as ahramin did, heat gun and heavy gloves. Didn't make a male mold just wrapped the floppy warm plastic around the underlying structure for shape. For a slow airplane one could just cold bend thin lexan (as sold in squares at Home Depot or Lowes etc) and screw it down over the hole. For a more elegant look on a faster plane, thicker acrylic. Mark's approach works well. Make a doubler to install inside, nut plates in the doubler, screw the heat shaped lens into the hole with spacers as needed to make it fit flush.