Any added cover on the braided plug lead is just to protect it from damage, not to add additional RF shielding. The braid does a good job of that all by itself.

I use 1/4" black molded black wire split lomb you can get at most any auto parts store, building supply, etc. This is retained by a single ty-wrap on each end of the cable. This gives excellent protection against the lead rubbing on anything in the airplane and the resulting vibration caused damage. It will not protect against high heat sources such as touching a muffler or pipe. For that I use a bit of silicone tubing in the area that needs the additional protection.

The Chinese ignitions come with a spiral wrap material that is popular but I don't care for the way it works so for these engines, it gets discarded in favor of the above material.