You may have to do some research/calling. See if you can contact the manufacturer of the ignition system (maybe RcExl) or the engine manufacturer.
They make caps to fit specific brands of plugs. I had a Champion plug in my engine, and I fought with RF noise problems for a very long time. Turned out the cap was made for an NKG plug (per RcExl). It fit, and no more RF noise!
Although the plugs were of the exact same type, it was very noticeable in side-by-side how differently constructed they were.