Are you a fun of wooden soap moulds? Or flexible soap moulds?
In Japan, many people use the milk container as a soap mould. It's not made of plastic as many other countries. We have 1000ml size milk and it's made of paper.Off course it does has coating inside.
You don't need any paper liner, you can peel off when soap gets dry. I used to use milk containers as a soap mould as well.
|Silicon Soap Mould|
So I'm using acrylic soap moulds now.