Addenda: A quick search found a number of sources for machinable elastomers.
For example (first google hit): http://waminco.com.au/materials
Another thing I wasn’t clear about: Holding the material during machining can be a challenge. When I talk about clamping the material between two plates, the plates may be premachined with the profile you are going to cut, plus a smidgen, so you are not contaminating the elastomer with aluminum chips.