I am about a 20 right now, and there are 2 carriers I use that fit well and are comfortable.
For a smaller baby, or in the winter, I use a Sleepy Wrap (basically the same as the Moby Wrap), but since I don't know the Moby's length I wouldn't know if it is long enough for a plus sized mom.
For my clip-on carrier, I use an Ergo Sport. I have a very large midsection, and there is plenty of length in the waistband for me to use it. I can even put it on unassisted, as long as I have loosened the upper strap enough to maneuver it once it is on.
You might want to check out Heavenly Hold to see what other options are available to you. You can check out their Facebook page. The owner is a mom who works from home and knows pretty much every baby carrier out there. I got my Ergo from her. https://www.facebook.com/#!/heavenlyhold or her website: www.HeavenlyHold.com
Good luck!