if you can't get the adapter to work without slipping, then there is something seriously wrong with the adapter or you're doing something seriously wrong.

I have 3 different adaptors, one has a completely smooth aluminum wedge, but that didn't cause my bars to slip at all. there might be too much grease or oil being used on the quill, or you're not tightening it enough.

The starnut on a threadless fork acts as a bearing pre-load adjuster, and it does NOTHING for quill adapters.
the top cap on adapters are really only there so it won't have this big gaping hole at the top of the fork.
Threaded forks rely on the threaded portion to adjust the bearing load.