Several people have come up with different ways to make a fuel cap assembly, but I have been using this very simple one for over 20 years and it is working just great and it is dirt cheap. Run down to your hardware store and get a 1-1/2 by 1-1/2 PVC threaded x slip fitting. Then go to the section of the store that has that little plastic nut that you use under your bathroom sink for the drain slip fitting. Finally, take a piece of .030" aluminum and make a little disc that fits into the slip fitting nut. Now you have all the pieces you need to make a fuel filler cap. I think the pictures should speak for themselves more than me cluttering up the airways with a bunch of verbiage.
A short length of PVC is used to connect the fuel filler to the main fuel tank.
Here in the same installation on my auxiliary fuel tank.