Build a dam around the feature you wish to copy. Use clay or hot glue. Leave 3/8 to 1/2 space around the feature & build depth appropriately. For a deeper mold could build a box using sheet styrene and hot glue it in place. Be aware of undercuts or tearout areas. You have now defined a positive space area.
Mix & pour RTV into the dammed area around the feature & allow to harden. Remove RTV from original piece. You now have a negative mold.
Mix & pour resin into the RTV mold. Allow to cure. Remove the positive part from negative mold.
Clean up part for use. Remove excess mold plug * over cast area.
That's the way I've done it in the past