I was Annelle (Steel Magnolias) at a community theatre with a similar problem. Annelle is quite pregnant in the last scene...
I used a rounded throw pillow and a tight undershirt... it rounded out the edges of the pillow and made it look quite real. I had people asking me during Meet and Greet how far I was along!!
It's all in the walk, too. Anyone can strap on a fake belly, but get someone to teach you to walk the talk!
And, Break a leg!!!