I’d had this problem rattling around in my head for a fair while – the baseplate at the bottom of the corner post, between the big windows, is much wider than the post itself. Since it sticks up about 30mm above the slab I needed some way to build the post out to be as fat as the baseplate, so the windows have something to fix into.
Ended up with a combination of brackets welded to the column, a timber outer face and timber blocks bolted to the brackets and the outer face screwed into them. Took a long time to put it all together (I’m not very fast or accurate at welding, or as I call it “sticky one metal thingie to the other metal thingie”) but it got there and seems to work. I think I’ll clad the outer face of this false column, instead of trying to render it.