First thing I replaced when I bought mine: Fuel Cap - Locking

 

Maybe price making one out of aluminum? Highly polished aluminum can look great and can stay looking great if you clear coat it with a 2k clear. Seems a pretty simple design. The trickiest bit would probably be the hinge.

 

So here is the issue, and reason I do not have it. When this type fails, you are hooped. You only find out when at the pump, empty, and that you can not remove it.

At least with the locking fuel door, worst case scenario is that you put a screwdriver to it an pry it open.
Wrecked gas lid, but at least you can fuel up. Not so with the one locked onto your filler neck. That is a tow to a garage, and in the end costs about the same as a locking fuel door.

And yes, I have seen these fail. A good reason as to why GM stopped selling them.

 

I never run my tank below 1/4, submerged in fuel keeps the pump cool preventing premature failure. My locking cap is over 10yrs old and still unlocks like new. It's not exposed to the elements behind the fuel door. Unlike my drivers door lock, have to silicone lube it every couple years to keep if free, just in case the fob fails.