Here's another non-maternity pattern, which would probably not need any adapting to make it fit a growing midriff! You could probably make a top like this from an unwanted circular skirt. Hang it round your neck and have someone cut armholes!
In fact, there is a free PDF pattern for a tent dress from Me Sew Crazy which you can download here. It's designed for knits, and so it won't perhaps look quite as stylish as the Cynthia Rowley pattern, but, as I said, it's free!
Alternatively, if you are confident enough, you could probably design a tent shape pattern yourself quite easily. You could make this a short top, or a full length dress, whatever you prefer. Here's how.
1) Take any sleeveless pattern with a neckline and armholes you like the shaping of. (You could make one with sleeves as well if you wished.) It's easiest with a pattern with a wide neck, and no vertical fitting darts. Here's an example. This one has just bust darts. You can ignore them, or include, as you wish.
4)Then draw a line of exactly the same length from the bottom of the armhole to whatever amount of flare you want. (The orange line.) I'd suggest you want to go out at least 20-25 degrees, and you could go more for an even more tent-y look!
5)Then you need to draw a nice curve from the bottom of this line to the bottom of the centre front line. I'm sorry my on-screen drawing talents are not so hot, I can do a much better job with a flat piece of paper. Of course, the growing bump will make the centre front rise, so you could drop the point for the centre front 4"-6" before drawing the curve.
(My drawing is getting worse, not better!)
7) Repeat for the back - but check you will have the same length for your side seams (after allowing for the bust dart in the front pattern piece, if any).
Just bear in mind that swing tops, like circular skirts, work best with a plain fabric, or one with a non-directional and non-geometric pattern.
Then you can cut out and make up your top as the pattern tells you.
I had a tent-style dress myself many years ago - below knee length. I wasn't pregnant, it was just the fashion. It looks a bit pinched on the waistline in this photo - my new husband's hand is round my waist! But this might have worked as a maternity dress, had it been needed at the time of the wedding! But this is some time before I became a New Mama, let alone a New Grandma.