It would be a good thing if the inside of the vent fit an Allen key or a slot on top to prevent it from spinning while tightening.

First there is one other alternative I have used, a tire valve stem, with a valve cap but you have to chase down a proper drill and piece of wire. There are youtube videos on this. These "magicfour fuel" caps are complete with everything you need. Only drill empty plastic containers. I was concerned about the plastic debris going inside the tank but the drill when slowly drilled expels the plastic up and out of the hole. The only problem is when the drill starts to go through the plastic and it