Inletting lockplates can be a real pain in the butt. I work on the assumption that it's better to do a good job on a simple design than a bad job on an ambitious design. So here's what I do: I make the plate, bend it to shape, then tape it in place. Then I take a Very Sharp, pointed artist's knife (an Xacto blade or scalpel will do) and cut into the stock, following the edge of the inlay precisely. Take your time and do it right. Then use a small chisel to deepen the knife-line, cutting straight down a few millimeters... I recommend using a mallet for fine chisel work rather than your hand... since a mallet struck with light blows is actually easier to control than shoving your entire are around.
When that's done, I chisel out a bit of wood using the scribed line as a stop.... this is basic chip-carving work, going from a few millimeters inside the cut, toward the periphery. When I have the border defined, I mount a flat router-bit in my moto-tool router and roughly reduce the height of all the stuff inside the scribed line to the required depth. When the area is cleared, I go back with a broad, sharp chisel and smooth down the surface to a consistent depth. Then I put the inlay into the hole and see if it fits. If it doesn't, I start trimming out bits till it does fit. Remember, if you make a mess of it, there's your chance to devise extra decorative bits to cover your mistakes... if it looks good when you finish, it ISN'T a mistake, it's a feature. Geezer