So my corset mock up is ugly. I said it was anyway, so here it is! Institutional mint and badly drawn lines everywhere! But look, a cape! And from just barely over 4m of fabric!
She has boobs! Yes they are OTT but you can see the places I am taking in and will also balance those puppies out 😉 This is also for an over bust which means it does look ridiculous! This is my base pattern to fit me and work with my idiosyncratic shape. There was a lot of experimentation in the 19thC for corsets so mine is a blend of genuinely conceivable pieces. I do only have four panels but may wind up cutting through the bust and angling to CF hem.
So, now the Elsa cape:
Super sturdy but fine tulle (photos show how transparent it is.) I cut the pattern over my Elissa underskirt to make sure I got the train as long as possible and forgot to account for it going over a pair of legs and a skirt.
1) I started by pinning the tulle to the form, selvage for the side/front seam and to see how long the gore would be on the diagonal, ending at the point which winds up side back of the train.
2) so this created a gore that I could top and tail (yay for reversible fabrics!) And so I smoothed it out by folding the fabric along the line and then cutting the first gore. 
3) I then laid that over the other side and cut and trimmed to match (if this was not reversible this would not work and I’d have two of the same sides)
4) So this creates the sides. I had measured to make sure there was enough length left that the back could be cut. This meant at least as long as the diagonal line plus any extra.
5) Success! Enough with a little left over. So I then pinned the remaining fabric to the back (cB on the fold and pinned at the hem as far to the selvage as possible) and pinned to best shape with the already cut pieces:
6) I laid the fabric on the fold and then used the same folding on the diagonal to smooth the line as before, matching the length to the side gore:
7) this left a narrow gore on either side. So rather than waste it I added it to the fonts and then reshaped all seams (smaller at the top tapering as wide as possible to the bottom).
I’m pretty sure there are five panels I just am not sure if I got them exactly where they are in the film (you see one straight to diagonal seam in the last scene on the fjord at least and the way it folds suggests seams) but i’m not too worried if not. This tulle is $14/m and I still have to dye it 😉 So I wanted best bang for buck!