Or -- you can bore a recess in the backplate, and turn a solid cylinder to fit the bore to make the pressure projection . Then screws through the cylinder and into the backplate to keep them firmly together.
For that matter -- drill and bore through the cylinder to allow workpieces to pass though the body and into the spindle. (I really should do that with mine -- though since it is a 6-jaw, I'm unlikely to be feeding stock through it. But someday, I probably will. :-)
Enjoy, DoN.