I settled on a paint stripper and a piece of scrap wood as a former, I did try all the other ideas but hot water I found was annoying the work area gets wet and the heat goes very quick. The Oven or Mirowave heated the whole work and bends were starting where I didn't want them.
Best advice take your time, bend slowly and keep checking.
I ended up with quite a lot of scrap, but hey plasticard is cheap!