I made my Fran out of craft foam too, but you may find some help in knowing how it was done.
Like the poster above, I used elastic to keep the thighs up. I made little foam loops inside, threaded elastic through, and popped a hook and eye on the end to join them. Easy to fasten, and they stay up all day.
The waist I presume means the little skirt she has. I just heat shaped the foam into a concertina, then glued it round the bottom of my corset. I did actually make chain mail to cover the sides, but that's very fiddly. I've seen people use textured fabric, so that's an option.
And as for the back... Yeah, this was a tricky one for me. In the end, I had the corset lace up at the back, then attached a back piece with Velcro to cover the join. That gets struck on last, and keeps it contoured to my back. It's got the little flappy tail on it too, which is cute.
Finished product - http://www.cosplay.com/costume/418391/