Changing stairs configuration

Hello all,

After some guidance as usual.

I've recently reconfigured a stringer staircase, or rather the walls around it.
The original setup meant that the 'open' side of the staircase was in the lounge, so we have had the wall moved to the other side of the staircase, meaning the 'open' side of the staircase is now in the hallway, probably the more normal way of things.
But, this leaves me with;
1. The bottom of the stringer that had the newel post originally, is now against a 'new' wall and the newel post has obviously been removed as it's no longer relevant.
2. The bottom of the stringer, that was originally against the, original but now removed wall, now needs a newel post.

Probably useful to add the subfloor is concrete

Hopefully, I've explained it sufficiently so I'm after options for fixing a new newel post to the stringer.
In my head I can visualise that once in place, the post can possibly move in 4 directions.
For arguments sake, back and forward, parallel to the stringer, and left and right, at 90 deg to the stringer.
Back and forward movement should be fixed by the banister rail
My concern is about the left and right movement.

Seeing as the newel is only fixed at the bottom, how do I make it most secure?

TIA