We don't see a lot of Original Halo 2 ODSTs, but I have always loved their look.
The files we have in The Armory have the Chest Pouches built into the chest armor:
This is a comprehensive zip file including numerous files from multiple artists. Contents of zip-file: halo-2-unsc-odst-chest.pdo Modeler: Ruze789 halo-2-unsc-odst-helmet.pdo Modeler: Ruze789 halo-2-unsc-odst-shin.pdo Modeler: Ruze789...
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This is due to the very low poly count of the Halo 2 models, where like the Halo CE Mark V and Marines, most of the details came from the textures rather than the actual base model itself. This does give you a good base as to what the size should be, but you will still have to free hand and fabricate a lot of the details seen on the game texture, but not captured in the model. As someone who does a Halo CE style Mark V, it's a pain I know all too well, but it's not an insurmountable obstacle. For example, the Halo CE Mark V has very similar pouches on its belt, but like the Halo 2 ODST, the model simply has them built in as bumps, with no detail.
The pouches can be fabricated out of foam, leather, or fabric, to get as close as possible in shape and the use of the one button snap fastener. Or you can try to find some similar pouches on Military Surplus or Airsoft website, however they won't be as accurate.
As for the helmet, if the original Halo 2 model in the The Armory entry above is not to your liking in that it is too low detail for you, the Halo 2 Anniversary version of the Helmet might be an option:
This is a comprehensive zip file including numerous files from multiple artists. Contents of zip-file: halo-2-anniversary-unsc-odst-helmet.pdo Modeler: Satchmo III PRINTABLE FILES halo-2-anniversary-unsc-odst-helmet.stl Modeler: Satchmo III
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