This reusable mold lining I used for a long time, is made of plastic sheets in A4 size they use for wrapping papers. Can be used for the acetate or polypropylene, these being the most resistant. Can be found at stationery stores or making photocopies and binding. It has a small mass problem sometimes stick a little jacket, but nothing that compromises the finish of the soap bar. To reuse simply wash and dry.
You can download the pdf document on downloads page – clik here