Hunters expand polystyrene in duck molds for decoys. They use closed and clamped molds and boil them in 55 gal drums...
Check it out:
http://ca.geocities.com/nbcc2@rogers.../epsdecoys.htm
You could use a lost wax process to make your molds,then do like the duck hunters to produce your foam positives. Seems like a lot of work, though, (when you could repeat your patterns in wax and use a lost wax / shell casting...)