I use a old Salton warming tray which provides the slow gentle heat you need. Then I use a heat gun very sparingly to adjust the facial expression I'm looking for. Once that is achieved I sandwich the duck between two pieces of plywood and put it in a vice for a few to 24 hours. Because different companies use different formulas for the plastic heating times are all over the place. Real rubber ducks like the Lanco don't melt. You have to be very patience, it takes a long time. Hope this helps. My "Pressed Duck" in this exhibit is made up of 101 unique ducks. Each duck is sandwiched between plexi glass which is held together with wing nuts. It took about two years to complete and you can see it on my web site:
http://www.lucysandersceery.net