He had some made up to suit his RCR and the "bolt" bit is long so you mount the spring bit on the inderside of the clip as opposed to on top.
When at speed the pressure builds up under the nostrils and will put massive strain on the bolts and spring holding it in - certainly worth looking at his option
As I was looking for the same fasteners as you are, I came to the conclusion, after I did some measurements on my car, that I needed the size 6 (larger ones) and .50 in length (EHF6-50).
I actually just ordered them here: Dzus Fasteners