*Sorry, I didn't know whether to put this in General or VL categories*

I've seen these on cars before, where people have them under their fuel flaps & also where people have welded a plate over their fuel cap hole (Shaved it) but installed one of these metal fuel caps instead. Now, my first question is, do these just screw into your original fuel hole, as a normal cap would, or do you need to fix them to your fuel hole via screws, allan screws etc, and question 2, can these be fitted to VL's at all?
I wouldn't mind getting one as the previous owner has bogged some rust inside my fuel filler flap and the fuel door hatch flap thing is hanging on by a thread, and also I like the race look that these caps have, plus I bought a new cap with a lock and left it on the fuel bowser so it's lost

Here's a link to the website I saw in Street Machine Magazine:

Newton Equipment

And here's a picture of the cap i'm referring to. well, the same style: