You can use an M5 screw or bolt. Cut the head off, thread the hole in the handlebars. Trim it to a length so it just screws into the bars enough to go through the wall but doesn't protrude into the bars where it will mess up the wiring. and screw it in using some locktight (round off the end of the bolt) You might need to drill out the receiving hole in the control a little.
I did this on mine when I replaced the controls which didn't have the studs. Worked great.
-P.