I'd make a pattern out of card stock. Take a piece of writing paper or computer paper if you don't write, lay it over the mortise of the inlet, and gently press the paper over the edges. Then draw along the creased lines with a pencil, and transfer the outline to the card stock. Cut it out with scissors or an x-acto knife. Try it in the inlet. Re-draw and cut it out as many times as necessary to make it a perfect fit. THEN trace it onto the brass. Remember, only half of the sideplate rests in the mortise...the other half will be raised above the wood, and there'll be a significant bevel along the edges...if it was that type of sideplate. I'm unfamiliar with the maker and his style, so just guessing.