Thanks for the compliment Crimson!
I found out after I made mine that FromTheBrinks' ODST armour files come with a template though I'm not sure I'd recommend buying the whole files just for this (even though they're really good files) I'm sure if you found a pepakura template you could separate this particular part out.
As for what I did, I just worked out the area to cover and spent ~half an hour fiddling around with the different shapes on paper until I had something that looked right. Then I transferred it over to a medium density (a little flexible) 4mm foam and 10mm for the parts along the spine. It's basically some shapes of foam on top of another sheet of foam.
Covered it in plastidip after that.
Depending on what you're using for the armoured section, you can attach it with hot glue and elastic (for flexibility and stretch) or contact cement (if you don't expect the area to flex).
LMK if you have any other questions 
