I don't really have any experience of casting resins though so I'm hoping someone could point me in the right direction. The real inserts are made from some sort of hard, machinable, plastic but I couldn't tell you which. They have no obvious flexibility in the hand but it's possible they might flex slightly when holding a persons full weight. One obvious difficulty I can see is with the centre of the heel area as the insert becomes quite thin, I'd guess about 2mm. Whatever I make the inserts out of I'd like to be able to work the final product (probably by sanding) to make it a custom fit for different pairs of shoes. Any advice is greatly appreciated.