3d print a part
Make a mold container (out of a plastic cup) and hot glue it to a support surface
Hot glue the part inside the container
Mix the two part resin in proper proportions
Pour it evenly and gently in the container to make the cast
Spray the mold with mold release
Mix iron powder with proper resin (ventilate, protect eyes and lungs) and add catalyst (dangerous)
Pour metal/resin mix gently and evenly in mold
When set, unmold
Tarnish and apply quick-rust
Tarnish again so the rust is only in the recessed details