The H&R kits (available in Road and Clubsport variants) are actually rather popular over here as a mid-range option.
Although the picture I attached above is not my pair, you're quite right regarding the missing piece. It's the nut/washer on top (997 343 561 90, item no 2 on the diagram linked above). As my top mounts are spotless I don't believe they have ever been installed on a car. I need to get a hold of a couple of those as well.
Just found a picture of a PSS9 installed with the stock top mount:
I guess that means that at least the MODE part would fit my coilovers since they and the PSS9's seem to have the same inner diameter as stock.