I do it myself and sell exclusive covers on tanks. The cover that you see is called the "United Kindom Aluminium+ Black". The lid is cast by me from a very high-quality aluminium. It uses stainless steel screws and nuts, oil-resistant rubber gaskets and a steel washer for installation in the tank.
The base of the lid and washer has a bottom base of 80mm (3.14inch). If you have a tank made of aluminum, then I for you cover the cover with an aluminium washer, which you can weld without trouble with a welding machine. The lid is completely handmade.
I personally polish it, drill it and polish it. Can be installed on a motorcycle tank or hot rod. To confirm my words, look at my photos in this lot.
If you have any questions about the installation, write to me. I try to send as soon as possible. It is possible to make an individual cover according to your sketches.The item "Custom Gas tank caps cover Harley, Bobber, Chopper, Cafe Racer, Oldschool" is in sale since Monday, April 22, 2019. This item is in the category "eBay Motors\Parts & Accessories\Motorcycle Parts\Body & Frame\Gas Tanks". The seller is "garage_nishtyaki" and is located in Energodar. This item can be shipped worldwide.