I came across a turtle shell that had been vacated by the former owner. My wife does plants and I wanted to make the shell into a planter by turning it upside down and mounting it on top of a short turned dowel that screws thru the bottom part of the turtle plate which is mounted on a finished wood base. The bottom plate (which I've seperated from the top shell) has a curve to it, turning up at the front. I want to soften it so I can flatten it to lay flat on the wood base. Any ideas for what it can be soaked in to soften it so I can flatten it?