from the ct cabinet to the ist main the Neutral is your bond,
also install a bonding strap or jumper from XO in CT cabinet to cabinet. (Usually supplied by manufacturer of Cabinet.
If not size a conductor using 250.66
Installing EGC would give you a parallel neutral as your also required to bond in MDP.
See Soares grounding book or goggle Soares info on net
http://iaeimagazine.org/magazine/1998/11/01/service-main-bonding-jumpers/