Actually, each marine gets his own herealdry, but it actually goes on a shield (a painted outilne of a shield, at least) and behind said field would be the justicar's fields and ordinaries.
Here, like this, using the heraldry I've come up with for my two justicars so far:
http://farm2.static.flickr.com/1232/737571078_e215e2ea37_o.jpg