The screw holes in the plastic ones are not at the correct distance, so they won't fit. If Ken Nash doesn't have the correct one, you might want to consider this: LTG member mavuser has a few Thunderbird truss rod covers from the correct period that he 'converted' to the style that you need. From a few feet away you won't be able to see the difference.
Here are some photos to give you an idea:
If that would work for you go ahead and shoot him a private message!