Here's a great article about blow molding from a few years ago: 

http://www.autospeed.com/cms/A_113285/article.html

The article takes you through Ian Linke's workshop, where he makes bubble canopies primarily for sailplanes. In addition to free blowing, he uses another technique called stretch molding. If you are interested in learning more about making bubble canopies, it is worth the read.