I'm 5'11 with terrible posture and I'm a size 18/20 (usually. Sometimes bigger by brand.). And what I've discovered is that if I feel good in it, I don't care. I've had a plan for a gibbous corset in my mind for a while and only just started it today. And I generally don't think corsets look good on girls with my body type and weight. But ya know what? I already feel hotter just knowing how cool the piece is. And I think with Gibbous, the CLOTHES are the point. It's wearable art. So it doesn't HAVE to be the most flattering thing in the world.
On a note that I think is more what you're looking for!
What flatters you is going to largely depend not on the simple fact that you have 'more cushion for the pushin', but the way said cushion is distributed.
STRAIGHT - If you're like me, you don't really curve. I'm like a tic-tac - - the same size all the way up and down! So I'm doing a corset to be able to cinch in my waist and make it curve a little more, instead of that more masculine up-and-down silhouette I'd get with anything else.
HOURGLASS - This is general curveyness. It's what I USED to be, and I'm workin' on it again! You don't want to shorten your frame, so stay away from any fabric or construction that makes the eye travel horizontally. Especially on and 'big and tall' girls, vertical or diagonal is the BOMB! Aside from that I think really anything can work on you if this is your shape. I would suggest, however, staying away from anything too loose. Like a flowy empire waist, which will just devour you. If you got it flaunt it!
PEAR - Tiny up top, wider on bottom. Of course. I'd suggest putting most of your detail across your boobs, if this is you. With a pear shape, smaller boobs and bigger butt. You want your boobs to look bigger or your butt smaller. There's not too much I can suggest for minimizing butt (I have an unfortunate lack of butt so I never dealt with that problem), but I know maximizing attention to boobage helps bring a pear closer to our middle-ground hourglass.
So for instance, with my corset, I'm doing the same basic construction all over, but I'm coming in with particularly lovely trim over the bust and shoulders. I have a brooch that will sit somewhere on my bust, haven't decided if I want it center yet or not. All that will make my boobs look bigger, which I don't need. HOWEVER, the construction pattern I'm using creates lines that will draw the eye of onlookers from my sides to my bellybutton - - an illusion trick to make myself look thinner. ((So good tip for bigger gibbous girls - USE LINES! Don't jumble it all up randomly. Have a secret agenda!)) That with the cinching of the corset will narrow my waist. Then the bottom of the corset is going to have some nice trim, and the top of my skirt is going to have some awesome beading or something done in almost horizontal diagonals to widen my hips.
See, it's all about illusions. I'm never gonna be skinny (unless i keep running, but that's beside the point!), but using lines and details to draw the eye where I want it to go, I can give the illusion that my body is one of the best danged plus size bodies there is! And that goes for ANY fashion, not just gibbous. A 'pile of rags' to use your example, can, when properly assembled, make ANY body type/weight look attractive. Just figure out what YOU need to accentuate to give the illusion you want.