My dear old mum requires inserts in her shoes for a problem with her feet. The inserts cost about £300 a pair and have to be custom made (by CNC) in the US. She can only afford a single pair but obviously she has more than a single pair of shoes which means a lot of messing about moving things around etc etc. Being the very kind son that I am 
I've offered to try and make copies of the originals for her.
Since I wasn't convinced I could successfully make copies I've gone for the cheap option initially and made a pair of two part molds out of plaster. I don't suppose they will last for more than a couple of castings but at this stage I'm looking more for a proof of concept than mass production. The molds I've made have sufficient detail that I'm confident that if I can get the correct combination of resin and release agent I can produce a workable product. So far I've been using Vaseline as a release agent and it seems to be working well.
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.