It looks nice and the switches are very tidy and clean...
I think I would have mounted them in a collar on the bar to put them in a vertical fashion / around the circumference of the collar... Then you could have one group of wires into the bar through a grommet. A chunk of aluminum and you could polish and have someone anodize it for you after all the fabrication. Small LEDs beside each button (towards the headstock) that are low output could help orient you at night... they would be invisible during the day and not distractingly bright at night.
Hardest part to fabricate it would be the centered hole that was a bit over 7/8" in diameter. Cut in half then drill the backside for recess for switches contacts. Drill the 3 holes for switches and 2 to 4 holes for clamping it to the bar and counter sink for screw heads (should only need 2 holes) before drilling the center hole would be the sequence I would use. Tap the 2 holes for clamp bolts after it is cut in half.
It could be fabricated using a drill press and router to round over the sides if you started with a cylindrical piece of aluminum.
The advantage of that approach would be the clutch perch would be much closer.
Here is my first attempt. At first I had a second thought about drilling the handlebars, so I got a pipe that slides firmly on handlebars then welded another pipe that cares the push buttons. As you can see I did not even think about drilling, so I made a cut through to slide the buttons.
If your are a good welder, you can give it a great shape. It requires you to plug the ends of the top pipe.
Then drill a tiny hole on handlebars to feed the 24 awg wires.
If money is not an issue, they are smaller buttons on Ebay for around $15 each. They require 9/32 holes (.2813 decimal inches), then use an M8x1.0 tap. If you have troubles finding a metric tap, you can use a 5/16-24 tap, which is the closest to the metric one.