I'd buy an 8" round FS mold. The 4" was too small for most of the applications I had in mind; a 3" wouldn't interest me at all for towers, but a couple of 2" or 3" pieces (but probably not a full mold's worth) might be great for adding rounded corners to stuff.

I did a couple of 1" diameter FS column short segments and quarter-circles (1/2" and 1" high), and they're proving to be useful for other places than I originally intended. A few 1/4 round segments like the Station Builder round pieces might be perfect. The more options the better.

And then there are "cladded" corners, with cut stone blocks holding the FS wall center in place.