I'm still going to do something like this (Photo below), but I'm going to do it next year after this ride hits the streets. I cannot make out from the photo how the mounts are fixed. It looks to me like there are aluminum brackets mounted to the roll bar (screwed in?). I can't make out the details on this, but I hardly noticed it until I zoomed in on it. It's a nice mounting. Notice his 3rd Brake Light too.
Thinking... use Nut Inserts. You could get by with 6-32 screws to mount the brackets.