One of my suppliers, Industrial Polymers has some stuff you need to look at. I highly recommend this company.
http://www.industrialpolymers.com
Check out thier expanding hydrospan.
" When parts cast from HydroSpan™ are soaked in water they accurately increase in size 60%. By making larger silicone molds from the expanded HydroSpan™ , you can produce a series of progressively larger molds. "
For minimum investment you can create small scale master parts - scale them up - then use there spray foam series for the final.