very good tutorials from punished props as well. I use scrap MDF (laser cut) for my molds now that I stock a lot of it (and makercase helps to build a box within seconds). Some people use lego bricks (or clones) to make the molds. I also like to use plastic card, ABS sheets or cintra PVC foam sheet. Feather cardboard (white) just works, it's just expensive IMO to make a mold, though anything hotglued can be re used to a certain extend since it's easy to remote the hot glue. However, once drenched with mold release, it's hard to glue again on waxed surfaces, that's where lego brick win
I'd generally put a few rubber bands around my makercase box in MDF and painters tape on the sides, and wax only the inside of the mold.