This will be long, it is the full description of how I do a 2 part mold with no vacuum or pressure. The piece I am making is the interior plate for the the 1/24 MPC 1966 Dodge Charger slot car. With this interior, you can convert any MPC Charger promo car to a perfect clone of the slot car issue as the bodies are the same colors, mold and cycolac plastic.
P1010007.JPG
P1010009.JPG
I start with covering the bottom with clay. I use the stuff in colors from Ben Franklin, I have used this clay over and over for years. Warm it and it works easier, I use a hair dryer. You want it to cover the plastic at least 3/8 inch for the lowest area.
I use a knife and trim it to the outline of the part as shown. A little taper as it goes down is preferred.
P1010010.JPG