A good way to jump into the resin casting pond is to go to an auto parts store and purchase JB Weld original epoxy and a tube of Loctite, Permatex or other high temperature 1200 degree exhaust gasket in a tube.
Take the small part ( such as a Santa Fe reefer hatch support) and coat it with the hi-temp silicone paste. Build up in thin layers, smoothing with a wetted finger after each coating. After your new mold is sized to your liking, mix up a small amount of J.B. Weld epoxy and fill the mold's cavity. J.B. Weld makes a good high quality casting.
I made a GE wheel slip axle-end generator years ago with this method. Each loco needed 4 of these parts and I got over 50 shots in the single mold, which is a remarkable result, as most RTVs are lucky to get 20.