I'm going to try the broom handle thing next, just for grins.

What I usually do is mount one lever where I want it. I then run a piece of tape up the inside curve of the bar bar from the bar end to the clamp and cut it to length. I then use the same piece of tape on the other side and put the clamp where the tape ends. It's generally close enough for my needs.

I used to eyeball it and generally close enough. Sometimes I'd be bugged by a mm of difference, so would just take the tape off one side and adjust as necessary and re-wrap the tape. A few minutes at the most, once per bike.