First you use a paper template to drill your button holes on the blank metal. Around the perimiter of the CP there were bolts that hold the plexi on. Just clamp your blank onto the CP to drill out those holes, then bolt it on. Use the blank CP with the button holes as a template to drill out the old CP. Once you're done, bolt it to the back of the old CP, glue it down with some industrial strength glue, and fill the top flush with bondo. Clean up the holes with sandpaper or the hole saw, then primer the whole thing viola', good as new.
this might not apply to your case, post a pic if you can.