As a machinist, I made my first one for my bike out of necessity, because I didn't get my paint saver sealed good when I painted my bike.
The top of the tanks on the choppers are fairly flat, but the K9's are more rounded. I found this out when I made one for Bluedog. So some modifications to the washers I have, may be needed to get it to lay flat on your tank. I made mine out of brass, But I have a stack of 308 Stainless Steel ones all ready polished. They are .100 thick, and I,ve been using a small bead of black silicone, instead of the rubber washer. You may have to take a razor blade to scrap the loose paint from around the fill hole.
I figure $30.00 for my labor and shipping is fair, for the lower 48 states. You can PM me if your interested.