I know rubber caps have been used for years. I have used them before with no problems. In fact I have no problem now when I get the cap shoved down properly.
Its just that I have that 100 installed on a 20 lb airplane. There is a vibration mode somewhere above idle that shakes that airplane like a dog shaking water off his back. I am afraid it might shake one of those plug caps loose.
I think I am going to fabricate a bracket to hold them in place. Maybe a tie wrap to the hose clamp on the plug base.
I guess the conductive or non conductive grease is a bad idea.
I am not flying or doing any outdoor testing now. It was 102 today - too hot for me