Screwed on plates are not what the CG had in mind when they say, "not easily removable". Most boats I have been on, the documentation number is routed into a structural beam or frame near the companionway or in the engine space. A cockpit locker lid (easily removable) may pass inspection, or it may not, depending on the mood of the inspector. Have the number routed into a board or fiberglass plate and fiberglass that onto a structural beam or frame out of sight in the engine space or under the companionway steps. This is for your safety, should the boat get stolen.