Not experience, just what I have read and seen.
i have looked into this myself. Mind you, I have never done it. I think I saw this on Utube. It is a material similar to a plastic cutting board and if I remember it is $8 a square ft. I went back and looked and it is called Melamine. It is a particle board product with a white coating on it used to make cabinets. It is of course rigid and they recommend using a product called Stoners mold release. In the video the guy used white glue on 3 sides of the vertical walls to prevent leaks and used screws on the last side to help to get the casting out. I hope at some point to try this and this seems the cheapest way. You will be able to configure the mold to fit your pressure pot or needs and you can make single blanks or a slab and cut on a table saw. I have also read about using PVC pipe. Mostly I read that this Stoners mold release will save a lot of headaches. When I get ready to try this I think I will try this method. Much cheaper than silicone. I hope this helps a bit. Let me know how it turns out for you.
Boswell