Measure how thick your lockplate is, and then plane the panel down to just 1/32 thicker than the bolster of the plate. Once you are all done with your inlet, you can plane off that last 1/32 which is all scarred up from inletting, edges rounded, etc, to leave a nice clean flat crisp pane face.
Eric, wait a minute before you get the plane out. Make sure your panel is parallel with the barrel flat. As you plane, be sure to keep the panel SQUARE with the top flat of the barrel.
Both you and I know how easy it is to get out of square and then you don't have enuff wood. Keep an eye always on your progress. Measure a lot.