When I swapped to a dual MC, I cut a hole big enough to access, welded a "lip" on the cut out, and 4 corner fastened it in. Here's a couple of pics...

http://img.photobucket.com/albums/v228/Avette4me/My%2058/Brakes/HoleforMasterCyl.jpg
http://img.photobucket.com/albums/v228/Avette4me/My%2058/Brakes/CoverforMaster-mockup.jpg

FWIW, the original plug still fits.