Regarding Dare's back:
Her underarmour looks like the standard ODST underarmour. The main difference is her backpack.
Aside from the imagery you can try and pull off of vid-docs (the new footage from the opening cinematic has some good angles of her), here's a quick sketch.
One thing you have to remember is the size. Dare's pack is much smaller than the standard ODST pack, closer in size to Mickey's comm pack.
The shape resembles a chopped down version of the standard ODST pack minus the side pouches.
Details include the panel that runs around the back and right side (note how it tapers off toward one side), and a little hooded keypad doohickey on the left.
===
For the chestpiece, Nightshade's ODST-game chestplate actually somewhat resembles Dare's chestplate. The main thing being it's too thin depth-wise compared to hers, which is an easy fix.
For stomach pieces, a quick rough scratch build can be done in less than an hour or two with some cardboard and foamies.
I should have a really rough Dare up in two days or so, with a lot of liberties and shortcuts taken (I'm leaning toward cutting out a lot of pieces for my sister's comfort) so you can take a look.
Best of luck on your costume! If I wasn't so swamped, I'd try and screencap some backpack images for you, but hopefully my quick doodle will help give you the general idea.