A set of the inverted kingpin Indys seemed to work well (and not loosen off at all) when I had the kingpin down as far into it as it would go, which was ok as the bushings were pretty much how I skated them anyway.
Other people I know seem to have had no end of trouble with the inverted kingpin assembly though, with the kingpin nut coming loose in the shaft, the kingpin not staying in place and one guy having to loosen off the kingpin to get the bushings right so it was almost out too far and sat up too high.
Thinking the right bushings with the kingpin down hard might be the answer, but it might be a struggle for some people to get it just right with the right bushings too.
I also tried (with success) two other options of kingpins, both with longer threads, so I could get it down lower and both stayed in place as well. They were old kingpins from Krux and Silver trucks, which had way more threads down the kingpins than the current Indy or some other brands do, so I am thinking that might be the best option to find other inverted kingpins to use in them, for people who can't get the Indy ones down far enough.