wot about some resistor-free NGK plug caps - that go straight onto the threaded bit?
They're the only ones I use, and they stay put. They also have grommety sleeves on lead-to-cap and cap to plug body.
And, . . as they are rigid it's impossible for the screw to 'miss' the HT cable core when you fit them. Which happens a lot with those soft rubbery ones if that's what you've got.
Cheap enough, and much easier than mashing solder, cross-threading on purpose or whatever?
Or do they lack 'the look'?