Ah, the top plate. Unless I'm missing something, it's not included in the Armory foam files. I've attached my copy of the chest plate foam file from 2016, but I have no idea when/where I got it.
This shows how the whole chest would come together:
1: halo-3-odst-unsc-odst-torso-foam
2: halo-3-odst-unsc-odst-torso-side-foam (mirrored to other side as well)
3: nightshade-chestplate-foam (attached to reply)
4: halo-3-odst-unsc-odst-chest-foam
As far as a guide, I'd recommend searching the forum for "ODST" and "Foam" to see other members running threads on their own builds. There are also threads covering the basics of foam, and how to use pepakura files for foam. Don't be afraid to pass through threads on non-ODST foam builds. Alot of the concepts transfer, and the Marine especially shares a similar construction.
Some examples:
Fallen/Random Ranger's Foam Guide:
Foam Guide
SierraEdict's First Build:
First Build - ODST - EVA Foam
Spidermonkey's Marine speed build (foam)
Foam - Halo 3 Marine Speedbuild
Fallen's Marine Pilot:
Foam - H3 Hornet Pilot - V1 Complete