Yes a mannequin can be a very useful tool. I am a rather large person, very peculiar shape, and I was wrapped in brown paper parcel tape and glue some 16 years ago now, so have a body shape that more or less was actually ME!!!! However I wanted to put this on over a rather old and battered dress dummy stand and initially opened her body up to sort of fill her out as much as possible and then padded over this opened up form. Big mistake. The dress form needed to be closed right up as small as it would go and then layers of wadding wrapped around her and secured by a stockingette cover before I put my double over the top and that did work. However since then I have sadly put on more weight and have had to add yet more wadding over the top of the paper parcel tape version of me, and in places have sewn the layers together to fill her out. I make a lot of toiles, so I used the latest one and pinned the front closed and then stuffed in more wadding where necessary so it rather looks as though it is straining (not a pretty sight at all) and I did this deliberately so I can pin tissue, calico or whatever into her and create the shapes I want knowing that if I do this tightly to her body that is my ease needed on me to be able to move, and so far this is working quite well. As she - her name is Matilda - is so well padded out she is rock solid, and the fact that I closed the original stand up nothing can fall through the gaps that would have been there if opened. I also have a chain necklace around her neck and this is there for a jewel neckline. I also have two separate calico arms that I have made which can be sewn into position for when I want them on her.
Of course I have doctored this to suit my needs, I have a few men's short sleeve thermal vests that have seen better days, i.e. wearing out, on her and these are sort of ribbed, so it is clear to see and create 'grain lines' and then my own idea was to stitch some lengths of net curtain string of weights to her, and these mark the CF, CB, the side seams, the shoulder princess seams, shoulder seam and I used the thickest string of weights I could find, and thus I can feel where these lines are when I am working with fabric on her and it really works for me.
So may I suggest if you have a garment that perhaps fits you exactly how you like it and is perhaps rather worn, to sacrifice this and put on your dress stand, to see just how much you are going to have to pad her out and where also use an old bra and that can be stuffed as well to suit your needs, and then get some of the lofty wadding that the quilters use and start padding her out to fit your garment, I found that these ribbed vests were excellent for holding the wadding in place, and as I have several on top of one another I have a lovely plump body to be able to pin into. At the moment I also have some weights dangling at the end of my string of weights, this is like a plumb line and helps hold this straight in the position I want the line to be in, and I just left them like that - nothing special, just bits and pieces that are reasonably weighty that I found around my home.
Do hope that some of this might inspire you to pad out your dress stands. At the moment Matilda is still dressed in her calico shoulder princess seamed top and I have a huge square floaty scarf thrown over the stop of the stand and this covers everything up - looks so much nicer than seeing just the calico top.