Yeah, for some reason the ball joint on the Slim is smaller than the regular TT.
You have 3 choices for moding it to work:
1) Put some super glue on the ball joint, stick the adapter on there and keep moving it until the glue dries. The added bulk by the glue should be enough to make it snug.
2) Put a little bit of cotton inside the adapter before sticking it on the ball joint.
3) Put a few coats of nail polish on the ball joint. (I think that using craft superglue that is applied with a brush would work pretty much the same.)
I tried the superglue trick first, and it did work. But the glue was a bit flaky and I didn't know how well it would hold up, so I've tried the nail polish trick. The glue scraped off really easy.
The only issue I have with the cotton trick is that it seems like you might end up having to futz with the cotton every time you take the head off. Nice thing is that it's a totally reversible mod. However, the nail polish mod would be reversible as well assuming that one could just get it off with normal nail polish remover.
Anyway, all of these mods retain articulation at the neck. I would do any of them rather than using putty to keep it fixed in place.