I'm looking to make some fiberglass motorcycle fenders of my own design. I'd like to be able to create the plug on the assembled bike so I can account for clearances. I was thinking the easiest way would be if I covered the front end in plastic, spray something like Great Stuff expanding foam all over the front tire and between the forks, then carve away. 

This seems like it would be much easier, cheaper and less messy than clay. Would the spray foam work well for this? Are there other similar alternatives?

thanks,
Dave