He used these Long Handlebar "spacegrips" by Minoura which you can get on Amazon for less than twenty dollars.
To get the horn to be right-side-up he needed to add some plastic, which you can see in the bottom picture.
I rotated the extension arm so that it points vertical, and slid it all the way up. The arm diameter is small, so I cut two pieces of an old bike tube, and placed it on the arm, along with the extra pad provided with the horn. The handlebar end has a clear rubber strip to protect the handlebars from scratching.
It took a little finaggling, since turning it upside down meant that the bottom of the snail & wiring were interfering with the handlebar extender mount. I wound up attaching a plastic pvc-like sleeve to make the bar extender longer.