I actually think a file for a back pack does not exist. however, scratch building it would be a lot easier and cheaper to do. here's a great reference video for all the back packs.here!
I agree with shaymin I don't think there is a pep file out there however it does not seem terribly hard to make. It will just take some ingenuity. As far as headstubs link the back pack there is for the snipers I beleive