just a little project to test with some plastic
I recommend using a clamp or press to keep the mold in place. you can put a hole in the side if you are using a metal cast to print.

XYZprinting

da Vinci mini w

Doesn't Matter

Doesn't Matter

100%

if you are using this one time, I recommend gluing the sides together and then prying them apart when the mold dries.
I also recommend making the infill 100% to help it withstand high pressure if you are going to use cardboard to make a paper mold.

Using a 2x4 brick PLA file from https://printablebricks.com, I imprinted it in two blocks and added a peg for sticking them together.