I assume you want to make cold process soap? If so then the type of plastic I would recommend for all steps of your soap making process, even the mould, is high-density polyethylene (aka HDPE) or polypropylene (aka PP).
This is not to be confused with polyethylene terephthalate (aka PET or PETE) type plastic (which is probably the most common household plastic like water bottles)
I do seem to remember seeing a new material come out for 3d printing as HDPE but I can't seem to find it atm, if I run across it again I will repost here for you!
The soap will not be very hot temperature wise when you pour it but as you probably know the caustic levels will not be skin safe for around 48-72 hours. So you also need to be very careful with what type of plastic you use so it can't eat into it or let it 'leak' bits into your soap.