I just purchased a F-250 with 6.7 diesel. The fuel door is plastic not a good place to attempt to install a fuel door lock. I also like the standard look. So I made my own DEF cap lock. Total cost $16. I had to file the DEF cap for it to fit in the hasp. I drilled a hole in the DEF cap for a Master lock. The hasp works like a "Club" to prevent being able to unscrew the DEF cap. I drilled a hole in the hasp and bolted in a small angle iron support which improves the difficulty to unscrew the DEF cap. But it is not needed, the Hasp prevents making a full turn off. Like any lock this can be defeated, but hopefully it will delay or cause a would be saboteur from completing his sabotage.I had a email conversation with an engineer at Stant. They are interested. They are working on it. The problem is that the DEF cap is small enough that the typical locking cap method won't work. If you have an idea, or better yet a prototype, I'm sure they would like to hear from you. Until then all we have is the locking door option.