Depends on a couple of things... are your cod and butt pieces able to connect together? I have some aluminum armor studs that have screw posts in them. My cod and butt pieces attach via the screw posts. If they are snug enough, they should stay in place pretty well. The girth belts helps to hold them in place as well.
If you do not have that option, you can use a snap system. Male snap attaches to the flightsuit and the female snap to the armor. The girth hides the seam between the cod and butt when using this method. You can also use some elastic. Female snaps on the edges of the cod and butt and male snaps on either end of a piece of elastic.
The back armor should connect with the collar piece via the armor studs. You shouldn't need to attach it to the vest as gravity and the jet pack help to hold it in place.