On a custom bike I like to have matching caps on gas tank & oil bag. In the new retro style, a young company called Nash Motorcycle offers this brass caps sold with weld-on vent fittings for venting tank into frame. Cap bung are machined out of mild steel for easy welding. You can order online at Nash Motorcycle Co. or call one of the 3 brothers (Trent, Taber or Teddy) at 360-693-4225.
Many of these embellishments are certainly well made and a delight for the eye.
A possible consideration for those that are building a bike or having one built would be the additional damage that they would cause you if your bike were to be involved in a collision. A gas cap of this design or any other protruding design just might collect some parts from your groin should you suddenly launch yourself across the gas tank.
Having been involved in a few collisions in my 47 years of riding I can attest to the advantages of a smooth launching ramp (gas tank).
Ouch!
i will second that goldiron the smoother the better