Interesting, I looked into using sonotubes for curved column faces too - kind of hard to find a place that carries the real big ones though. I'm planning on something sort of Sandman-esque as far as column shape (rectangular back box, with a curved face / speaker grille) - but more of a gentle curve, like 1/4 slice from a larger sonotube vs. 1/2. Also pondering getting curved perforated metal to do the trick, but could get pricey (but would hold the fabric up nicely).
Maybe the inward bow on the fabric is due to too much tension horizontally - I'm thinking you'd want it stretched tightly vertically, but pretty loose horizontally.