I've used the SL7 bearings and the SL7 compression ring. I've sanded the edged of the cap so it sits flush. I think I did al the right things. It compresses fine at first, just slips a tiny bit after a couple of harsh gravel roads.
I had some creaking in the headset so I loosened everything up to grease up the bearing, but then for the life of me couldn’t get it back to a position that didn’t give me headset play.
I ended up combining two hobbies I never thought would combine - woodworking and bike, and clamped the bottom of the form and the stem, then tightened the stem, then the expander plug and it’s fixed it.
Perhaps give that a shot? Below is a picture.