As you can see by the way I mounted these brackets, the side leg of the Tee needed to wrap around the handlebars for a more secure fit. I used a bandsaw to make roughly a 30 degree cut. I started nearly flush with the edge of the pipe and cut to about the midway point of the side leg.
I wanted the side leg of the Tee to rest completely and tightly around the handlebars. The bars were slightly larger than 1/2" so the tee didn't really fit tightly. One at a time I used a heat gun (or hair dryer) to heat the cut side leg until soft, then pressed it onto my handlebars. Hold it until it cools and hardens or spray a little water on it to cool. This made a tight custom fit. You can see in the photo that the side leg has been spread a little from its original size.