Use the two small screws to attach the alligator clip to the pen. I used to epoxy with the screw to ensure a good seal.
I used epoxy to glue on the rubber padding to the clip as well. There is probably a better glue out there to use but at the moment, I'm not aware of it. I had to put a good coating on there to keep it attached.
In the second image you can see the valve open and the hole it's covering. The scraping marks are from me sanding the top to make sure the surface was still flush. After drilling the hole the edges were a little rough.