If you use solid wood, you'll either have to find a very wide (and expensive) board to make the dutchman, or you will have to run the grain of the patch perpendicular to the grain of the panel, which will be very noticeable. Also, since the gash seems rather long, you may have expansion/contraction issues using solid wood, since the plywood won't move but the solid wood will. Instead, I'd use an offcut of the walnut ply that you have, or get a small sheet of walnut veneer to put over some birch plywood. If you use veneer over a piece of plywood that's thinner than the original panel. then you can route the shape of the slot to within a few thou of the thickness of the patch ply, and just sand it down to the exact depth. Using a planer to level a patch to thin-veneered hardwood ply is very dangerous (aesthetically) IMO unless you are supremely confident in your planing abilities.