I quit using polypropylene long time ago. I have been using fiberglass head which is much easy to build and more robust. My ACDC (.050" thick) and Minnie (.030") both uses fiberglass head. I bought the material from ACP Composites.
http://www.acp-composites.com/acp-fs.htm
http://www.deltronix.net/cgi/acp_display.exe
Wahid