Ask and you shall recieve! (Flip Top Gas cap Adapter)
It is too bad that you are getting hosed by the machine shops in your area. Just thinking about this idea... What if you could find someone out here who is a machinist or would be willing to partner up with you on this. Even if you had to raise the price, you may not have to raise it up so much if you could get someone to hook up with to produce these adapters. Probably not feasible but just a thought.